Game Maker Server doesn't support GMS2 currently, the developer has been working on a re-write of it for about 2 years now.
You're best off making your own networking functions, or using a method that won't go out of date. As by the time he gets it working with GMS2, something else will probably break it.
I used it for GMS1.4, and it worked quite well, but it still isn't as good as becoming well acquainted with networking and making your own system specifically for your game. And you'll have control over everything then.
Your own GameMaker Networking api is your own main goal instead of that website yet.
I really suggest not to officially make those online multiple player video games. Unless you will want to share your own first PC video games with the same 2 players on a 1 keyboard only. But be aware if you have google search your really own real GameMaker Studio 2 api, then the most very basic Networking api part a little bit more later on with those really officially made yoyo video games document are much more harder than you think to make those very most basic multiple server online video games player instead. You might mind as you will need to know some of the very basic server backend science side as well. Such as what is the buffers, raw, ext, socket, config, and the most basic packet side as well. http://docs2.yoyogames.com/