The short version, then the thing that catches people out.
rtmp://ingest.global-contribute.live-video.net/app
Most guides still give the server as rtmp://live.twitch.tv/app. That address is legacy. The current one is below, and it comes from Twitch’s own ingest list rather than from a blog post.
Paste the key into Beamd and the server URL is filled in for you. Add your Twitch username as well and you get viewer counts, plus Beamd will stop the stream if the channel goes offline instead of paying to broadcast to nobody.
Your key is stored encrypted and is never shown back to you, on screen or in email.
Stream 24/7 with your computer off