Web Messaging Guest API: 400 Bad Request on typing indicator payload

Trying to push typing indicators from our custom widget to Genesys Cloud using the Guest API. Calling POST /api/v2/guest/conversations/webmessaging/{id}/typing with this JSON:

{"conversationId": "123-abc", "typing": true}

Getting a 400 Bad Request. The docs are vague on the exact schema requirements for the typing endpoint. Is the payload structure wrong, or do I need a specific header? Tried removing the conversationId but that just gives a 404. Running this from Sydney.