You will need to use send itent action.
Action: android.intent.action.SEND
MIME typ: text/plain
Extra1 name (first row): android.intent.extra.TEXT
Extra1 value (second row): your custom text
The shared text needs to be inside ""
So this intent would share a simple text. The problem is that this intent won't open the classic share menu, but instead will behave as if you want to open the text and not share it, hence opening the "open in app" menu, which may not be what you want. If you check 2. answer on this thread on stack overflow it mentions use of createChooser method instead of just only starting the activity and I think this is maybe something we can't overcome on the user side with what MacroDroid offers.
@MacroDroidDev may know more.
I have used Sharing-type intents before, such as: Intent intent = new Intent(Intent.ACTION_SEND); intent.setType("plain/text"); intent.putExtra(Intent.EXTRA_EMAIL, new String[] { "---" }); intent.
stackoverflow.com
EDIT: The shared text doesn't need to be inside "".