POST /api/v2/conversations/cobrowse returns 403 Forbidden

Trying to start a cobrowse session via the Conversations API. Sending this payload to /api/v2/conversations/cobrowse:

{
 "type": "cobrowse",
 "to": [{ "id": "user-123" }],
 "from": { "id": "agent-456" }
}

Getting a 403 Forbidden. The token has cobrowse:control scope. What am I missing?