System.Diagnostics.Process.GetCurrentProcess().Kill();
I sure hope you added the onClick behaviour to your button. Else, I recommend this, add a script to the canvas, define add the button to a public definition of a button, like this :-
public Button butt;
And in the Start() function, add-
butt.onClick.AddListener(stopper);
and outside the start and update functions, create a private function stopper()
stopper(){
System.Diagnostics.Process.GetCurrentProcess().Kill();
}