How to use properly Facebook SDK to send text message with Messenger? I found similar question but there aren't any answer Android. Send text to facebook messenger by Facebook SDK 4.X Also I read Messenger documentation https://developers.facebook.com/docs/messenger/android but there I coundn't find the answer.
The second link you supplied makes clear there are only a limited number of supported mime types, the first link also illustrates there is no support for text/plain.
The SDK is for sharing alternative content on messenger, it's not for allowing creation of new messenger clients or bots
So the TL;DR is: You cannot send text messages