Guest API POST /api/v2/conversations/messages returning 400 Bad Request

Getting a 400 Bad Request when trying to send a message via the Guest API without the widget. I’m passing the conversationId and message in the JSON body as shown below, but it keeps failing validation.

{
 "conversationId": "abc-123",
 "message": "test"
}

The docs say this is valid for programmatic guests. Am I missing a header?