POST /api/v2/conversations/messaging/... 400 Bad Request on canned response

Hey folks,

I’m trying to send a canned response from our WFM tool during an active chat. I’m trying to send a message to the participant. The body is:

{"type": "text", "text": "Thanks for waiting."}

I get a 400 error saying participantId is invalid. The ID comes from the webhook event. Am I missing a header or is this endpoint wrong for canned replies?