Opens a stream that shows a short code for signing the caller in on another device or origin. Requires a signed-in user session; API keys, service tokens and verification tokens are refused. The first frame is offered with an 8-character Crockford code and a link that carries it in the URL fragment. A fresh code is offered every 60 seconds, and each code is accepted for 120 seconds, so the current and previous codes both work. The stream ends with redeemed once one of its codes is used, or ended after 10 minutes. Closing the stream early does not revoke its codes; they expire on their own.