YouTube is the easiest destination: the key never changes.

Before you start

YouTube requires your channel to be verified with a phone number. Do this at youtube.com/verify if you haven't already.
Live streaming has to be enabled once, and YouTube can take up to 24 hours to switch it on. Do this at least a day before your first show.
YouTube's rules: you must be 16 or older and have no live-streaming strikes in the last 90 days.
Use a computer (laptop or desktop). The steps below don't work in the YouTube mobile app.

Get your Stream URL and Stream key (once)

On a computer, go to studio.youtube.com and sign in to the Google account that owns your channel.
Click Create (top right), then Go live. If YouTube asks you to enable live streaming, do it, then come back once it's approved.
In the Live Control Room, click the Stream tab (the icon that looks like a small broadcast tower). If this is your first time, fill in a title and click Create stream.
Look for the Stream settings box. You'll see two rows: Stream URL and Stream key (paste in encoder).
Open the Stream key dropdown. YouTube starts you on Auto-generated key, which it mints fresh every session — pick Create new stream key instead, call it Backstage Pass, and select it. That is the key you paste below, and the one to choose every show: YouTube only shows video arriving on the key selected here.
In BackstagePass, open Studio → Destinations and press Connect next to YouTube.
Back in YouTube, click Copy next to Stream URL and paste it into the Stream URL box. It's normally rtmp://a.rtmp.youtube.com/live2.
Click Copy next to Stream key, paste it into the Stream key box, and press Save.
Destinations confirms the address and key are saved and well-formed — never which key YouTube is listening on, so your first GO LIVE is the real test.

What to do each time you go live, in this order.

On YouTube. Do this first, so your video has somewhere to land.

Click + Create at the top, then Go live.
You land on the Stream tab. Check the Key dropdown shows Backstage Pass. Nothing to copy. Leave this window open.

In the control room, check YouTube shows next to Ready on at the top right. Then press GO LIVE.

Back in the YouTube Studio window.

Within a few seconds your video shows in the preview and the status changes to live. If YouTube shows a Go live button instead, press it.

End on BackstagePass, then check the stream has ended in YouTube Studio. If it's still running, press End stream.

Your key is reusable. Your saved key stays the same for every show, so you only make it once — but YouTube still starts the Stream key dropdown on Auto-generated key, so pick yours there before each show. If you ever think someone else has it, click the stream key name, choose Manage stream keys and create a new one. Then, in BackstagePass, open Manage next to YouTube on Destinations, press Replace key, paste the new one, and press Save new key.
Set your title and privacy in YouTube. Each stream uses the title, description and privacy (Public, Unlisted or Private) saved in the Live Control Room. Update them there before the show.
YouTube also makes a vertical copy. By default YouTube publishes a 9:16 crop of your stream to the Shorts feed alongside the normal 16:9 version. Turn off Dual stream in Stream settings if you don't want this.
Recordings are kept automatically. YouTube saves streams under 12 hours to your channel when you end them.
On by default. Once your key is saved, YouTube's page on Destinations has an On by default switch. When this is on, YouTube is already ticked every time you create a new live. You can still untick it for any single live.

Every platform, and the one step most people miss: Connect your destinations.