I'm trying to install steamcmd on Debian 9. I believe it's this package: https://packages.debian.org/stretch/steamcmd
However I get a "package not found" error when using "apt-get install" because I need to add the non-free repository. I tried adding "non-free" to every line of the repository file, I tried adding new ones I found on internet but nothing is working, plus I found steamcmd on many places and I have no idea if one's better than another or that one is official, like these for example that I'm not even sure if they're the same:
Can anyone tell help me getting this in the apt-get repository so I can install it?