api: Add sendSubmessage.

Co-authored-by: Isham Mahajan <[email protected]>

Author: WesleyAC

src/api/index.js

@@ -40,6 +40,7 @@ import toggleMobilePushSettings from './settings/toggleMobilePushSettings';
 import createStream from './streams/createStream';
 import getStreams from './streams/getStreams';
 import updateStream from './streams/updateStream';
+import sendSubmessage from './submessages/sendSubmessage';
 import getSubscriptions from './subscriptions/getSubscriptions';
 import subscriptionAdd from './subscriptions/subscriptionAdd';
 import subscriptionRemove from './subscriptions/subscriptionRemove';
@@ -86,6 +87,7 @@ export {
   createStream,
   getStreams,
   updateStream,
+  sendSubmessage,
   getSubscriptions,
   setTopicMute,
   subscriptionAdd,

src/api/submessages/sendSubmessage.js

@@ -0,0 +1,13 @@
+/* @flow strict-local */
+
+import type { ApiResponse, Auth } from '../transportTypes';
+import { apiPost } from '../apiFetch';
+
+/** See https://zulip.readthedocs.io/en/latest/subsystems/widgets.html#poll-todo-lists-and-games */
+// `msg_type` only exists as widget at the moment, see #3205.
+export default async (auth: Auth, messageId: number, content: string): Promise<ApiResponse> =>
+  apiPost(auth, 'submessage', {
+    message_id: messageId,
+    msg_type: 'widget',
+    content,
+  });