Alrightly, this guide is for people that's seeks for a lill fps optimization.
We all know that most of games that use the Unity engine doesn't use the true fullscreen option or the developers just doesn't add this option directly in the game for some reasons, anyways let's start.

It's pretty easy : right click on the game-------->proprieties-------->set launch options---------->and write : -window-mode exclusive

It should now launch your game in true fullscreen when you will start your game, enjoy !