Auto-authenticating to iframe-embedded Kibana dashboard

If you are using Shield 2.3, and you have the front-end plugin installed in Kibana, then yes, this is possible. It doesn't work as you've put in your example.

Instead, pass a valid Basic Authorization header with the request will cause Shield will validate the session, creating and using an authorization cookie in the background.

Note that prior to 2.3, I don't believe this was possible at all.