Guest API POST /conversations/{id}/messages returning 400 Bad Request

Trying to implement the Guest API to bypass the Messenger widget entirely. The token generation works fine, but sending a message fails. Here is the POST to /api/v2/guest/conversations/{id}/messages:

{
 "from": { "id": "guest-123" },
 "text": "Hello"
}

Getting a 400 with message “Invalid message payload”. The from object seems required but I’m not sure if the ID format is correct. What am I missing here?