They seem to have things pretty well locked down. But I managed to accomplish this with a combination of AutoShare and AutoInput plugins for tasker.
Create a day and/or time based profile.
Create a task with the following actions:
Create AutoShare Action (plugin > Autoshare)
use the find compatible apps option and select Messenger from the list.
Then set the text to share to "Good morning!".
Add a wait action and set it for 2 seconds (this is so the Messenger screen will load)
Create an AutoInput Action action (plugin > AutoInput Action) with the following settings:
- set the type to "Text"
- set the action to "click"
- set the Value to the group name (ie. Group1).
Create another AutoInput Action action with the following settings:
- Action: Click
- Field Type: Id
- Field Id: com.facebook.orca:id/send_button
This is optional:
You can have tasker go back to the home screen by adding the action: App > Go Home
You can also kill the app by add the action: App > Kill App > choose "Messenger" in the list.
Create a notification that will let you know the message was sent.