It depends on what you're calling. If you are trying to go to a new scene, then you could use this:
SceneManager.call(Scene_Menu)
Replace Scene_Menu with the name of the scene you are calling.
If you want to do something else (or even if you want to do that but need to do some other things first), then you just do it. Notably, however, the context for anything you are doing is the Game_Interpreter class. Thus, if you want to do something particular, or complicated, you could do it as a method of that class.
If what you are looking to do is just create a window, then the window should ultimately be located within the Scene_Map class. So, you should make a method in that class doing it, and then you can call it with something like:
$scene.method
Where method is the name of the method you created.