diff --git a/.env.dist.testing b/.env.dist.testing
index 48cc810..ca3522d 100644
--- a/.env.dist.testing
+++ b/.env.dist.testing
@@ -1,5 +1,6 @@
 CONVERTKIT_API_BROADCAST_ID="8697158"
 CONVERTKIT_API_FORM_ID="2765139"
+CONVERTKIT_API_FORM_ID_2="2780977"
 CONVERTKIT_API_LEGACY_FORM_URL="https://app.convertkit.com/landing_pages/470099"
 CONVERTKIT_API_LANDING_PAGE_URL="https://cheerful-architect-3237.ck.page/cc5eb21744"
 CONVERTKIT_API_LEGACY_LANDING_PAGE_URL="https://app.convertkit.com/landing_pages/470103"
diff --git a/.env.example b/.env.example
index 83d8a88..c138bea 100644
--- a/.env.example
+++ b/.env.example
@@ -7,6 +7,7 @@ CONVERTKIT_OAUTH_CLIENT_SECRET=
 CONVERTKIT_OAUTH_REDIRECT_URI="https://convertkit-github.local/wp-admin/options-general.php?page=_wp_convertkit_settings"
 CONVERTKIT_API_BROADCAST_ID="8697158"
 CONVERTKIT_API_FORM_ID="2765139"
+CONVERTKIT_API_FORM_ID_2="2780977"
 CONVERTKIT_API_LEGACY_FORM_URL="https://app.convertkit.com/landing_pages/470099"
 CONVERTKIT_API_LANDING_PAGE_URL="https://cheerful-architect-3237.ck.page/cc5eb21744"
 CONVERTKIT_API_LEGACY_LANDING_PAGE_URL="https://app.convertkit.com/landing_pages/470103"
diff --git a/.github/workflows/tests.yml b/.github/workflows/tests.yml
index 43d8299..4894c77 100644
--- a/.github/workflows/tests.yml
+++ b/.github/workflows/tests.yml
@@ -42,7 +42,7 @@ jobs:
       # Make sure your committed .env.dist.testing file ends with a newline.
       # The formatting of the contents to include a blank newline is deliberate.
       - name: Define GitHub Secrets in .env.dist.testing
-        uses: DamianReeves/write-file-action@v1.1
+        uses: DamianReeves/write-file-action@v1.3
         with:
           path: .env.dist.testing
           contents: |
@@ -68,6 +68,11 @@ jobs:
       - name: Build PHP Autoloader
         run: composer dump-autoload
 
+      # Run PHPStan for static analysis.
+      - name: Run PHPStan Static Analysis
+        working-directory: ${{ env.PLUGIN_DIR }}
+        run: php vendor/bin/phpstan analyse --memory-limit=1250M
+
       # Run Coding Standards.
       - name: Run Coding Standards
         run: php vendor/bin/phpcs --standard=phpcs.xml
diff --git a/composer.json b/composer.json
index e2c5048..9b46196 100644
--- a/composer.json
+++ b/composer.json
@@ -23,7 +23,7 @@
     "vlucas/phpdotenv": "^5.5",
     "phpunit/phpunit": "^5.7 || ^9.0",
     "squizlabs/php_codesniffer": "^3.3",
-    "phpstan/phpstan": "^1.2"
+    "phpstan/phpstan": "^2.0"
   },
   "autoload": {
     "psr-4": {
diff --git a/phpstan.neon.dist b/phpstan.neon.dist
index c3bd077..77033b3 100644
--- a/phpstan.neon.dist
+++ b/phpstan.neon.dist
@@ -6,4 +6,4 @@ parameters:
 
     # Should not need to edit anything below here
     # Rule Level: https://phpstan.org/user-guide/rule-levels
-    level: 8
\ No newline at end of file
+    level: 10
\ No newline at end of file
diff --git a/src/ConvertKit_API.php b/src/ConvertKit_API.php
index 86b1888..6fcd9d3 100644
--- a/src/ConvertKit_API.php
+++ b/src/ConvertKit_API.php
@@ -142,6 +142,7 @@ private function create_log(string $message)
         // Mask email addresses that may be contained within the message.
         $message = preg_replace_callback(
             '^[_a-z0-9-]+(\.[_a-z0-9-]+)*@[a-z0-9-]+(\.[a-z0-9-]+)*(\.[a-z]{2,3})^',
+            // @phpstan-ignore-next-line - see https://github.com/phpstan/phpstan/issues/10396
             function ($matches) {
                 return preg_replace('/\B[^@.]/', '*', $matches[0]);
             },
@@ -176,7 +177,7 @@ public function get_oauth_url(string $redirectURI)
      * @param string $authCode    Authorization Code, returned from get_oauth_url() flow.
      * @param string $redirectURI Redirect URI.
      *
-     * @return array<string, int|string> API response
+     * @return mixed|array<string, int|string> API response
      */
     public function get_access_token(string $authCode, string $redirectURI)
     {
@@ -214,7 +215,7 @@ public function get_access_token(string $authCode, string $redirectURI)
      * @param string $refreshToken Refresh Token.
      * @param string $redirectURI  Redirect URI.
      *
-     * @return array<string, int|string> API response
+     * @return mixed|array<string, int|string> API response
      */
     public function refresh_token(string $refreshToken, string $redirectURI)
     {
@@ -337,7 +338,7 @@ public function get_resource(string $url)
      *
      * @throws \Exception If JSON encoding arguments failed.
      *
-     * @return false|mixed
+     * @return mixed|object
      */
     public function request(string $endpoint, string $method, array $args = [])
     {
diff --git a/src/ConvertKit_API_Traits.php b/src/ConvertKit_API_Traits.php
index b5fc993..fb54712 100644
--- a/src/ConvertKit_API_Traits.php
+++ b/src/ConvertKit_API_Traits.php
@@ -161,7 +161,7 @@ public function get_growth_stats(\DateTime $starting = null, \DateTime $ending =
      *
      * @see https://developers.convertkit.com/v4.html#convertkit-api-forms
      *
-     * @return false|array<int,\stdClass>
+     * @return mixed|array<int,\stdClass>
      */
     public function get_forms(
         string $status = 'active',
@@ -198,7 +198,7 @@ public function get_forms(
      *
      * @see https://developers.convertkit.com/v4.html#convertkit-api-forms
      *
-     * @return false|array<int,\stdClass>
+     * @return mixed|array<int,\stdClass>
      */
     public function get_landing_pages(
         string $status = 'active',
@@ -222,21 +222,57 @@ public function get_landing_pages(
         );
     }
 
+    /**
+     * Adds subscribers to forms in bulk.
+     *
+     * @param array<array<string,string>> $forms_subscribers_ids Array of arrays comprising of `form_id`, `subscriber_id` and optional `referrer` URL.
+     * @param string                      $callback_url          URL to notify for large batch size when async processing complete.
+     *
+     * @since 2.1.0
+     *
+     * @see https://developers.kit.com/v4.html#bulk-add-subscribers-to-forms
+     *
+     * @return false|object
+     */
+    public function add_subscribers_to_forms(array $forms_subscribers_ids, string $callback_url = '')
+    {
+        // Build parameters.
+        $options = ['additions' => $forms_subscribers_ids];
+        if (!empty($callback_url)) {
+            $options['callback_url'] = $callback_url;
+        }
+
+        // Send request.
+        return $this->post(
+            'bulk/forms/subscribers',
+            $options
+        );
+    }
+
     /**
      * Adds a subscriber to a form by email address
      *
      * @param integer $form_id       Form ID.
      * @param string  $email_address Email Address.
+     * @param string  $referrer      Referrer.
      *
      * @see https://developers.convertkit.com/v4.html#add-subscriber-to-form-by-email-address
      *
      * @return false|mixed
      */
-    public function add_subscriber_to_form_by_email(int $form_id, string $email_address)
+    public function add_subscriber_to_form_by_email(int $form_id, string $email_address, string $referrer = '')
     {
+        // Build parameters.
+        $options = ['email_address' => $email_address];
+
+        if (!empty($referrer)) {
+            $options['referrer'] = $referrer;
+        }
+
+        // Send request.
         return $this->post(
             sprintf('forms/%s/subscribers', $form_id),
-            ['email_address' => $email_address]
+            $options
         );
     }
 
@@ -245,6 +281,7 @@ public function add_subscriber_to_form_by_email(int $form_id, string $email_addr
      *
      * @param integer $form_id       Form ID.
      * @param integer $subscriber_id Subscriber ID.
+     * @param string  $referrer      Referrer URL.
      *
      * @see https://developers.convertkit.com/v4.html#add-subscriber-to-form
      *
@@ -252,9 +289,20 @@ public function add_subscriber_to_form_by_email(int $form_id, string $email_addr
      *
      * @return false|mixed
      */
-    public function add_subscriber_to_form(int $form_id, int $subscriber_id)
+    public function add_subscriber_to_form(int $form_id, int $subscriber_id, string $referrer = '')
     {
-        return $this->post(sprintf('forms/%s/subscribers/%s', $form_id, $subscriber_id));
+        // Build parameters.
+        $options = [];
+
+        if (!empty($referrer)) {
+            $options['referrer'] = $referrer;
+        }
+
+        // Send request.
+        return $this->post(
+            sprintf('forms/%s/subscribers/%s', $form_id, $subscriber_id),
+            $options
+        );
     }
 
     /**
@@ -456,7 +504,7 @@ public function get_sequence_subscriptions(
      *
      * @see https://developers.convertkit.com/v4.html#list-tags
      *
-     * @return false|array<int,\stdClass>
+     * @return mixed|array<int,\stdClass>
      */
     public function get_tags(
         bool $include_total_count = false,
@@ -863,10 +911,26 @@ public function get_subscriber_id(string $email_address)
             ['email_address' => $email_address]
         );
 
+        if (!$subscribers instanceof \stdClass) {
+            return false;
+        }
+
+        if (!is_array($subscribers->subscribers)) {
+            return false;
+        }
+
         if (!count($subscribers->subscribers)) {
             return false;
         }
 
+        if (!$subscribers->subscribers[0] instanceof \stdClass) {
+            return false;
+        }
+
+        if (!is_int($subscribers->subscribers[0]->id)) {
+            return false;
+        }
+
         // Return the subscriber's ID.
         return $subscribers->subscribers[0]->id;
     }
@@ -878,7 +942,7 @@ public function get_subscriber_id(string $email_address)
      *
      * @see https://developers.convertkit.com/v4.html#get-a-subscriber
      *
-     * @return false|integer
+     * @return mixed|integer
      */
     public function get_subscriber(int $subscriber_id)
     {
@@ -895,7 +959,7 @@ public function get_subscriber(int $subscriber_id)
      *
      * @see https://developers.convertkit.com/v4.html#update-a-subscriber
      *
-     * @return false|mixed
+     * @return mixed
      */
     public function update_subscriber(
         int $subscriber_id,
@@ -930,7 +994,7 @@ public function update_subscriber(
      *
      * @see https://developers.convertkit.com/v4.html#unsubscribe-subscriber
      *
-     * @return false|object
+     * @return mixed|object
      */
     public function unsubscribe_by_email(string $email_address)
     {
@@ -949,7 +1013,7 @@ public function unsubscribe_by_email(string $email_address)
      *
      * @see https://developers.convertkit.com/v4.html#unsubscribe-subscriber
      *
-     * @return false|object
+     * @return mixed|object
      */
     public function unsubscribe(int $subscriber_id)
     {
@@ -967,7 +1031,7 @@ public function unsubscribe(int $subscriber_id)
      *
      * @see https://developers.convertkit.com/v4.html#list-tags-for-a-subscriber
      *
-     * @return false|array<int,\stdClass>
+     * @return mixed|array<int,\stdClass>
      */
     public function get_subscriber_tags(
         int $subscriber_id,
@@ -1044,7 +1108,7 @@ public function get_broadcasts(
      *
      * @see https://developers.convertkit.com/v4.html#create-a-broadcast
      *
-     * @return false|object
+     * @return mixed|object
      */
     public function create_broadcast(
         string $subject = '',
@@ -1103,7 +1167,7 @@ public function create_broadcast(
      *
      * @see https://developers.convertkit.com/v4.html#get-a-broadcast
      *
-     * @return false|object
+     * @return mixed|object
      */
     public function get_broadcast(int $id)
     {
@@ -1118,7 +1182,7 @@ public function get_broadcast(int $id)
      *
      * @see https://developers.convertkit.com/v4.html#get-stats
      *
-     * @return false|object
+     * @return mixed|object
      */
     public function get_broadcast_stats(int $id)
     {
@@ -1151,7 +1215,7 @@ public function get_broadcast_stats(int $id)
      *
      * @see https://developers.convertkit.com/#create-a-broadcast
      *
-     * @return false|object
+     * @return mixed|object
      */
     public function update_broadcast(
         int $id,
@@ -1213,7 +1277,7 @@ public function update_broadcast(
      *
      * @see https://developers.convertkit.com/v4.html#delete-a-broadcast
      *
-     * @return false|object
+     * @return mixed|object
      */
     public function delete_broadcast(int $id)
     {
@@ -1266,7 +1330,7 @@ public function get_webhooks(
      *
      * @throws \InvalidArgumentException If the event is not supported.
      *
-     * @return false|object
+     * @return mixed|object
      */
     public function create_webhook(string $url, string $event, string $parameter = '')
     {
@@ -1340,7 +1404,7 @@ public function create_webhook(string $url, string $event, string $parameter = '
      *
      * @see https://developers.convertkit.com/v4.html#delete-a-webhook
      *
-     * @return false|object
+     * @return mixed|object
      */
     public function delete_webhook(int $id)
     {
@@ -1389,7 +1453,7 @@ public function get_custom_fields(
      *
      * @see https://developers.convertkit.com/v4.html#create-a-custom-field
      *
-     * @return false|object
+     * @return mixed|object
      */
     public function create_custom_field(string $label)
     {
@@ -1409,7 +1473,7 @@ public function create_custom_field(string $label)
      *
      * @see https://developers.convertkit.com/v4.html#bulk-create-custom-fields
      *
-     * @return false|object
+     * @return mixed|object
      */
     public function create_custom_fields(array $labels, string $callback_url = '')
     {
@@ -1444,7 +1508,7 @@ public function create_custom_fields(array $labels, string $callback_url = '')
      *
      * @see https://developers.convertkit.com/v4.html#update-a-custom-field
      *
-     * @return false|object
+     * @return mixed|object
      */
     public function update_custom_field(int $id, string $label)
     {
@@ -1463,7 +1527,7 @@ public function update_custom_field(int $id, string $label)
      *
      * @see https://developers.convertkit.com/#destroy-field
      *
-     * @return false|object
+     * @return mixed|object
      */
     public function delete_custom_field(int $id)
     {
@@ -1510,7 +1574,7 @@ public function get_purchases(
      *
      * @see https://developers.convertkit.com/v4.html#get-a-purchase
      *
-     * @return false|object
+     * @return mixed|object
      */
     public function get_purchase(int $purchase_id)
     {
@@ -1535,7 +1599,7 @@ public function get_purchase(int $purchase_id)
      *
      * @see https://developers.convertkit.com/v4.html#create-a-purchase
      *
-     * @return false|object
+     * @return mixed|object
      */
     public function create_purchase(
         string $email_address,
diff --git a/tests/ConvertKitAPITest.php b/tests/ConvertKitAPITest.php
index b6839a3..9de4f49 100644
--- a/tests/ConvertKitAPITest.php
+++ b/tests/ConvertKitAPITest.php
@@ -95,6 +95,9 @@ protected function setUp(): void
             clientSecret: $_ENV['CONVERTKIT_OAUTH_CLIENT_SECRET'],
             accessToken: $_ENV['CONVERTKIT_OAUTH_ACCESS_TOKEN']
         );
+
+        // Wait a second to avoid hitting a 429 rate limit.
+        sleep(1);
     }
 
     /**
@@ -762,8 +765,8 @@ public function testGetGrowthStatsWithStartDate()
         $this->assertArrayHasKey('ending', $stats);
 
         // Assert start and end dates were honored.
-        $this->assertEquals($stats['starting'], $starting->format('Y-m-d') . 'T00:00:00-04:00');
-        $this->assertEquals($stats['ending'], $ending->format('Y-m-d') . 'T23:59:59-04:00');
+        $this->assertEquals($stats['starting'], $starting->format('Y-m-d') . 'T00:00:00-05:00');
+        $this->assertEquals($stats['ending'], $ending->format('Y-m-d') . 'T23:59:59-05:00');
     }
 
     /**
@@ -799,8 +802,8 @@ public function testGetGrowthStatsWithEndDate()
         $this->assertArrayHasKey('ending', $stats);
 
         // Assert start and end dates were honored.
-        $this->assertEquals($stats['starting'], $starting->format('Y-m-d') . 'T00:00:00-04:00');
-        $this->assertEquals($stats['ending'], $ending->format('Y-m-d') . 'T23:59:59-04:00');
+        $this->assertEquals($stats['starting'], $starting->format('Y-m-d') . 'T00:00:00-05:00');
+        $this->assertEquals($stats['ending'], $ending->format('Y-m-d') . 'T23:59:59-05:00');
     }
 
     /**
@@ -2699,6 +2702,261 @@ public function testGetTagSubscriptionsWithInvalidTagID()
         $result = $this->api->get_tag_subscriptions(12345);
     }
 
+    /**
+     * Test that add_subscribers_to_forms() returns the expected data.
+     *
+     * @since   2.1.0
+     *
+     * @return void
+     */
+    public function testAddSubscribersToForms()
+    {
+        // Create subscriber.
+        $emailAddress = $this->generateEmailAddress();
+        $subscriber = $this->api->create_subscriber(
+            email_address: $emailAddress
+        );
+
+        // Set subscriber_id to ensure subscriber is unsubscribed after test.
+        $this->subscriber_ids[] = $subscriber->subscriber->id;
+
+        // Add subscribers to forms.
+        $result = $this->api->add_subscribers_to_forms(
+            forms_subscribers_ids: [
+                [
+                    'form_id' => (int) $_ENV['CONVERTKIT_API_FORM_ID'],
+                    'subscriber_id' => $subscriber->subscriber->id,
+                ],
+                [
+                    'form_id' => (int) $_ENV['CONVERTKIT_API_FORM_ID_2'],
+                    'subscriber_id' => $subscriber->subscriber->id,
+                ],
+            ]
+        );
+
+        // Assert no failures.
+        $this->assertCount(0, $result->failures);
+
+        // Confirm result is an array comprising of each subscriber that was created.
+        $this->assertIsArray($result->subscribers);
+    }
+
+    /**
+     * Test that add_subscribers_to_forms() returns the expected data
+     * when a referrer URL is specified.
+     *
+     * @since   2.1.0
+     *
+     * @return void
+     */
+    public function testAddSubscribersToFormsWithReferrer()
+    {
+        // Create subscriber.
+        $emailAddress = $this->generateEmailAddress();
+        $subscriber = $this->api->create_subscriber(
+            email_address: $emailAddress
+        );
+
+        // Set subscriber_id to ensure subscriber is unsubscribed after test.
+        $this->subscriber_ids[] = $subscriber->subscriber->id;
+
+        // Add subscribers to forms.
+        $result = $this->api->add_subscribers_to_forms(
+            forms_subscribers_ids: [
+                [
+                    'form_id' => (int) $_ENV['CONVERTKIT_API_FORM_ID'],
+                    'subscriber_id' => $subscriber->subscriber->id,
+                    'referrer' => 'https://mywebsite.com/bfpromo/',
+                ],
+                [
+                    'form_id' => (int) $_ENV['CONVERTKIT_API_FORM_ID_2'],
+                    'subscriber_id' => $subscriber->subscriber->id,
+                    'referrer' => 'https://mywebsite.com/bfpromo/',
+                ],
+            ]
+        );
+
+        // Assert no failures.
+        $this->assertCount(0, $result->failures);
+
+        // Confirm result is an array comprising of each subscriber that was created.
+        $this->assertIsArray($result->subscribers);
+
+        // Assert referrer data set for subscribers.
+        foreach ($result->subscribers as $subscriber) {
+             $this->assertEquals(
+                 $subscriber->referrer,
+                 'https://mywebsite.com/bfpromo/'
+             );
+        }
+    }
+
+    /**
+     * Test that add_subscribers_to_forms() returns the expected data
+     * when a referrer URL with UTM parameters is specified.
+     *
+     * @since   2.1.0
+     *
+     * @return void
+     */
+    public function testAddSubscribersToFormsWithReferrerUTMParams()
+    {
+        // Define referrer.
+        $referrerUTMParams = [
+            'utm_source'    => 'facebook',
+            'utm_medium'    => 'cpc',
+            'utm_campaign'  => 'black_friday',
+            'utm_term'      => 'car_owners',
+            'utm_content'   => 'get_10_off',
+        ];
+        $referrer = 'https://mywebsite.com/bfpromo/?' . http_build_query($referrerUTMParams);
+
+        // Create subscriber.
+        $emailAddress = $this->generateEmailAddress();
+        $subscriber = $this->api->create_subscriber(
+            email_address: $emailAddress
+        );
+
+        // Set subscriber_id to ensure subscriber is unsubscribed after test.
+        $this->subscriber_ids[] = $subscriber->subscriber->id;
+
+        // Add subscribers to forms.
+        $result = $this->api->add_subscribers_to_forms(
+            forms_subscribers_ids: [
+                [
+                    'form_id' => (int) $_ENV['CONVERTKIT_API_FORM_ID'],
+                    'subscriber_id' => $subscriber->subscriber->id,
+                    'referrer' => $referrer,
+                ],
+                [
+                    'form_id' => (int) $_ENV['CONVERTKIT_API_FORM_ID_2'],
+                    'subscriber_id' => $subscriber->subscriber->id,
+                    'referrer' => $referrer,
+                ],
+            ]
+        );
+
+        // Assert no failures.
+        $this->assertCount(0, $result->failures);
+
+        // Confirm result is an array comprising of each subscriber that was created.
+        $this->assertIsArray($result->subscribers);
+
+        // Assert referrer data set for subscribers.
+        foreach ($result->subscribers as $subscriber) {
+            $this->assertEquals(
+                $subscriber->referrer,
+                $referrer
+            );
+            $this->assertEquals(
+                $subscriber->referrer_utm_parameters->source,
+                $referrerUTMParams['utm_source']
+            );
+            $this->assertEquals(
+                $subscriber->referrer_utm_parameters->medium,
+                $referrerUTMParams['utm_medium']
+            );
+            $this->assertEquals(
+                $subscriber->referrer_utm_parameters->campaign,
+                $referrerUTMParams['utm_campaign']
+            );
+            $this->assertEquals(
+                $subscriber->referrer_utm_parameters->term,
+                $referrerUTMParams['utm_term']
+            );
+            $this->assertEquals(
+                $subscriber->referrer_utm_parameters->content,
+                $referrerUTMParams['utm_content']
+            );
+        }
+    }
+
+    /**
+     * Test that add_subscribers_to_forms() returns the expected errors
+     * when invalid Form IDs are specified.
+     *
+     * @since   2.1.0
+     *
+     * @return void
+     */
+    public function testAddSubscribersToFormsWithInvalidFormIDs()
+    {
+        // Create subscriber.
+        $emailAddress = $this->generateEmailAddress();
+        $subscriber = $this->api->create_subscriber(
+            email_address: $emailAddress
+        );
+
+        // Set subscriber_id to ensure subscriber is unsubscribed after test.
+        $this->subscriber_ids[] = $subscriber->subscriber->id;
+
+        // Add subscribers to forms.
+        $result = $this->api->add_subscribers_to_forms(
+            forms_subscribers_ids: [
+                [
+                    'form_id' => 9999999,
+                    'subscriber_id' => $subscriber->subscriber->id,
+                ],
+                [
+                    'form_id' => 9999999,
+                    'subscriber_id' => $subscriber->subscriber->id,
+                ],
+            ]
+        );
+
+        // Assert failures.
+        $this->assertCount(2, $result->failures);
+        foreach ($result->failures as $failure) {
+            $this->assertEquals(
+                $failure->errors[0],
+                'Form does not exist'
+            );
+        }
+    }
+
+    /**
+     * Test that add_subscribers_to_forms() returns the expected errors
+     * when invalid Subscriber IDs are specified.
+     *
+     * @since   2.1.0
+     *
+     * @return void
+     */
+    public function testAddSubscribersToFormsWithInvalidSubscriberIDs()
+    {
+        // Create subscriber.
+        $emailAddress = $this->generateEmailAddress();
+        $subscriber = $this->api->create_subscriber(
+            email_address: $emailAddress
+        );
+
+        // Set subscriber_id to ensure subscriber is unsubscribed after test.
+        $this->subscriber_ids[] = $subscriber->subscriber->id;
+
+        // Add subscribers to forms.
+        $result = $this->api->add_subscribers_to_forms(
+            forms_subscribers_ids: [
+                [
+                    'form_id' => (int) $_ENV['CONVERTKIT_API_FORM_ID'],
+                    'subscriber_id' => 999999,
+                ],
+                [
+                    'form_id' => (int) $_ENV['CONVERTKIT_API_FORM_ID_2'],
+                    'subscriber_id' => 999999,
+                ],
+            ]
+        );
+
+        // Assert failures.
+        $this->assertCount(2, $result->failures);
+        foreach ($result->failures as $failure) {
+            $this->assertEquals(
+                $failure->errors[0],
+                'Subscriber does not exist'
+            );
+        }
+    }
+
     /**
      * Test that add_subscriber_to_form_by_email() returns the expected data.
      *
@@ -2731,6 +2989,118 @@ public function testAddSubscriberToFormByEmail()
         );
     }
 
+    /**
+     * Test that add_subscriber_to_form_by_email() returns the expected data
+     * when a referrer is specified.
+     *
+     * @since   2.1.0
+     *
+     * @return void
+     */
+    public function testAddSubscriberToFormByEmailWithReferrer()
+    {
+        // Create subscriber.
+        $emailAddress = $this->generateEmailAddress();
+        $subscriber = $this->api->create_subscriber(
+            email_address: $emailAddress,
+        );
+
+        // Set subscriber_id to ensure subscriber is unsubscribed after test.
+        $this->subscriber_ids[] = $subscriber->subscriber->id;
+
+        // Add subscriber to form.
+        $result = $this->api->add_subscriber_to_form_by_email(
+            form_id: (int) $_ENV['CONVERTKIT_API_FORM_ID'],
+            email_address: $emailAddress,
+            referrer: 'https://mywebsite.com/bfpromo/',
+        );
+
+        $this->assertInstanceOf('stdClass', $result);
+        $this->assertArrayHasKey('subscriber', get_object_vars($result));
+        $this->assertArrayHasKey('id', get_object_vars($result->subscriber));
+        $this->assertEquals(
+            get_object_vars($result->subscriber)['email_address'],
+            $emailAddress
+        );
+
+        // Assert referrer data set for form subscriber.
+        $this->assertEquals(
+            $result->subscriber->referrer,
+            'https://mywebsite.com/bfpromo/'
+        );
+    }
+
+    /**
+     * Test that add_subscriber_to_form_by_email() returns the expected data
+     * when a referrer is specified that includes UTM parameters.
+     *
+     * @since   2.1.0
+     *
+     * @return void
+     */
+    public function testAddSubscriberToFormByEmailWithReferrerUTMParams()
+    {
+        // Define referrer.
+        $referrerUTMParams = [
+            'utm_source'    => 'facebook',
+            'utm_medium'    => 'cpc',
+            'utm_campaign'  => 'black_friday',
+            'utm_term'      => 'car_owners',
+            'utm_content'   => 'get_10_off',
+        ];
+        $referrer = 'https://mywebsite.com/bfpromo/?' . http_build_query($referrerUTMParams);
+
+        // Create subscriber.
+        $emailAddress = $this->generateEmailAddress();
+        $subscriber = $this->api->create_subscriber(
+            email_address: $emailAddress,
+        );
+
+        // Set subscriber_id to ensure subscriber is unsubscribed after test.
+        $this->subscriber_ids[] = $subscriber->subscriber->id;
+
+        // Add subscriber to form.
+        $result = $this->api->add_subscriber_to_form_by_email(
+            form_id: (int) $_ENV['CONVERTKIT_API_FORM_ID'],
+            email_address: $emailAddress,
+            referrer: $referrer,
+        );
+
+        $this->assertInstanceOf('stdClass', $result);
+        $this->assertArrayHasKey('subscriber', get_object_vars($result));
+        $this->assertArrayHasKey('id', get_object_vars($result->subscriber));
+        $this->assertEquals(
+            get_object_vars($result->subscriber)['email_address'],
+            $emailAddress
+        );
+
+        // Assert referrer data set for form subscriber.
+        $this->assertEquals(
+            $result->subscriber->referrer,
+            $referrer
+        );
+        $this->assertEquals(
+            $result->subscriber->referrer_utm_parameters->source,
+            $referrerUTMParams['utm_source']
+        );
+        $this->assertEquals(
+            $result->subscriber->referrer_utm_parameters->medium,
+            $referrerUTMParams['utm_medium']
+        );
+        $this->assertEquals(
+            $result->subscriber->referrer_utm_parameters->campaign,
+            $referrerUTMParams['utm_campaign']
+        );
+        $this->assertEquals(
+            $result->subscriber->referrer_utm_parameters->term,
+            $referrerUTMParams['utm_term']
+        );
+        $this->assertEquals(
+            $result->subscriber->referrer_utm_parameters->content,
+            $referrerUTMParams['utm_content']
+        );
+    }
+
     /**
      * Test that add_subscriber_to_form_by_email() throws a ClientException when an invalid
      * form ID is specified.
@@ -2792,6 +3162,110 @@ public function testAddSubscriberToForm()
         $this->assertEquals(get_object_vars($result->subscriber)['id'], $subscriber->subscriber->id);
     }
 
+    /**
+     * Test that add_subscriber_to_form() returns the expected data
+     * when a referrer is specified.
+     *
+     * @since   2.1.0
+     *
+     * @return void
+     */
+    public function testAddSubscriberToFormWithReferrer()
+    {
+        // Create subscriber.
+        $subscriber = $this->api->create_subscriber(
+            email_address: $this->generateEmailAddress()
+        );
+
+        // Set subscriber_id to ensure subscriber is unsubscribed after test.
+        $this->subscriber_ids[] = $subscriber->subscriber->id;
+
+        // Add subscriber to form.
+        $result = $this->api->add_subscriber_to_form(
+            form_id: (int) $_ENV['CONVERTKIT_API_FORM_ID'],
+            subscriber_id: $subscriber->subscriber->id,
+            referrer: 'https://mywebsite.com/bfpromo/',
+        );
+
+        $this->assertInstanceOf('stdClass', $result);
+        $this->assertArrayHasKey('subscriber', get_object_vars($result));
+        $this->assertArrayHasKey('id', get_object_vars($result->subscriber));
+        $this->assertEquals(get_object_vars($result->subscriber)['id'], $subscriber->subscriber->id);
+
+        // Assert referrer data set for form subscriber.
+        $this->assertEquals(
+            $result->subscriber->referrer,
+            'https://mywebsite.com/bfpromo/'
+        );
+    }
+
+    /**
+     * Test that add_subscriber_to_form() returns the expected data
+     * when a referrer is specified that includes UTM parameters.
+     *
+     * @since   2.1.0
+     *
+     * @return void
+     */
+    public function testAddSubscriberToFormWithReferrerUTMParams()
+    {
+        // Define referrer.
+        $referrerUTMParams = [
+            'utm_source'    => 'facebook',
+            'utm_medium'    => 'cpc',
+            'utm_campaign'  => 'black_friday',
+            'utm_term'      => 'car_owners',
+            'utm_content'   => 'get_10_off',
+        ];
+        $referrer = 'https://mywebsite.com/bfpromo/?' . http_build_query($referrerUTMParams);
+
+        // Create subscriber.
+        $subscriber = $this->api->create_subscriber(
+            email_address: $this->generateEmailAddress()
+        );
+
+        // Set subscriber_id to ensure subscriber is unsubscribed after test.
+        $this->subscriber_ids[] = $subscriber->subscriber->id;
+
+        // Add subscriber to form.
+        $result = $this->api->add_subscriber_to_form(
+            form_id: (int) $_ENV['CONVERTKIT_API_FORM_ID'],
+            subscriber_id: $subscriber->subscriber->id,
+            referrer: $referrer,
+        );
+
+        $this->assertInstanceOf('stdClass', $result);
+        $this->assertArrayHasKey('subscriber', get_object_vars($result));
+        $this->assertArrayHasKey('id', get_object_vars($result->subscriber));
+        $this->assertEquals(get_object_vars($result->subscriber)['id'], $subscriber->subscriber->id);
+
+        // Assert referrer data set for form subscriber.
+        $this->assertEquals(
+            $result->subscriber->referrer,
+            $referrer
+        );
+        $this->assertEquals(
+            $result->subscriber->referrer_utm_parameters->source,
+            $referrerUTMParams['utm_source']
+        );
+        $this->assertEquals(
+            $result->subscriber->referrer_utm_parameters->medium,
+            $referrerUTMParams['utm_medium']
+        );
+        $this->assertEquals(
+            $result->subscriber->referrer_utm_parameters->campaign,
+            $referrerUTMParams['utm_campaign']
+        );
+        $this->assertEquals(
+            $result->subscriber->referrer_utm_parameters->term,
+            $referrerUTMParams['utm_term']
+        );
+        $this->assertEquals(
+            $result->subscriber->referrer_utm_parameters->content,
+            $referrerUTMParams['utm_content']
+        );
+    }
+
     /**
      * Test that add_subscriber_to_form() throws a ClientException when an invalid
      * form ID is specified.
@@ -5086,11 +5560,11 @@ private function callPrivateMethod($obj, $name, array $args)
      *
      * @since   1.0.0
      *
-     * @param   string $domain     Domain (default: convertkit.com).
+     * @param   string $domain     Domain (default: kit.com).
      *
      * @return  string
      */
-    private function generateEmailAddress($domain = 'convertkit.com')
+    private function generateEmailAddress($domain = 'kit.com')
     {
         return 'php-sdk-' . date('Y-m-d-H-i-s') . '-php-' . PHP_VERSION_ID . '@' . $domain;
     }