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.

GET /api/v2/conversations/messaging/guests works fine with the token. But POST to /conversations/messaging/guests/{id}/messages 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"}