Guest API POST returning 401 when widget is disabled

We’re building a custom chat UI so I disabled the standard messenger widget. Now trying to post messages via the Guest API.

Getting the guest details works fine with the token. But trying to send an inbound open message fails with 401 Unauthorized.

The token is valid for an hour. Is there a specific scope missing for sending? Or is the guest ID format wrong?

{"text": "test", "type": "text"}