400 error on Slack webhook from Auth0

I’m trying to setup a monitoring stream which posts to slack if login attempts are failing, etc. I’ve created the webhook in Slack, and the Slack-provided test POST requests successfully generate posts on Slack. However, Auth0 doesn’t seem to be able to post to Slack. The Health tab shows a 400 error with:
“message”: “Bad Request”,
“details”: “missing_text_or_fallback_or_attachments”

or

“details”: “invalid_payload”

I’ve tried setting the content format to JSON Array, JSON Lines, and JSON Object. I’m guessing none of these are the correct format for Slack- is there an easy way to get the data in the correct format to be digested by the Slack webhook?