Endpoint / method to post a reply to a message

New users to our community often post questions that are incomplete -- not enough detail for our expert users to post a meaningful reply.  I would like to implement a "Needs more info" button that our Super Users can click on the topic root message that will create a boilerplate reply, perhaps from a generic moderator account, so that our Super Users don't feel the need to coax/cajole more details out of the OP.

I think the way to implement this is via an endpoint.  Am I on the right track?

I've figured out how to add an action button to just the root message.  What I'd like is:

• an example of an endpoint that posts/replies or otherwise modifies an existing message
• an example of the action script that I would add to the button to call that endpoint...and ideally refresh the message so that the person who clicked it would see the change applied.

Thanks,
Chris