The short version, then the thing that catches people out.
rtmp://a.rtmp.youtube.com/live2
YouTube gives you two kinds of key, and that control at the top of the video window is a dropdown, so it is easy to be on the wrong one. A key belonging to a scheduled broadcast works once, for that broadcast. "Default stream key" is reusable, which is what a 24/7 channel needs. If your stream worked yesterday and refuses to connect today, you almost certainly copied the single-use one.
With Beamd you can skip all of this: connect your YouTube account and it reads the reusable key straight from the API, so there is nothing to copy and nothing to keep track of.
Your key is stored encrypted and is never shown back to you, on screen or in email.
Stream 24/7 with your computer off