Type this in your browser: steam://run/10000

sadly this is the only official steam demo of it, not the actual game.

It's actually steam://run/10010 for Demo 1
or steam://run/10050 for Demo 2
Not sure what's the difference between the two.
What you linked is for the full game not the demo and won't install unless you own it.

Oh, I probably typed wrong.

But I have installed both demos now.

Maybe there is a command to buy the actual game?

Maybe there is a command to buy the actual game?
That's sadly not possible, the packages are disabled, nowdays they even disable free packages so games that are getting removed these days even if they're free or have a demo you can't download it anymore unless you activate them before they are taken off the store.

I installed Demo 1. Ingame I was asked to download Demo 2. I agreed and Demo 2 was installed.

I have an installable .iso of the game but don't know if I can upload it, in my eyes this game is abandonware so it shouldn't be a problem, you can go to archive dot org and there's also ISOs there.

You don't need to install the demo, for full game;
https://discordapp.com/invite/tX7Buk9

Myabandonware & SplashDamage Official Website has the game.
Just google the name of the game, and then add "myabandonware" in the search, about SplashDamage official website, is unclear whether it's a demo or the full game.
https://www.splashdamage.com/games/enemy-territory-quake-wars/

Maybe there is a command to buy the actual game?
There was. But they patched it this may and refunded people money, some even got bans for that.