Any **custom** webhook for Mattermost returns error 400

Summary

Any custom webhook for Mattermost returns error 400:

{"id":"web.incoming_webhook.text.app_error","message":"No text specified","detailed_error":"","request_id":"ig3n7qehnbdwtpdcpezotaspwc","status_code":400}

Normal Mattermost integration works fine.

Steps to reproduce

Just add a custom webhook via "Add webhook" and try to test it.

Maybe I'm doing something wrong, but I just need to separate bots to different channels for merge requests and for issues/comments, so MRs logs don't crowd normal channel discussion.

I thought a custom hook (in addition to normal MM integration) would do, but this 400 error puzzles me and Google does not help either.

Cheers.