I've been implementing this into my mod and I wanna ask, how do I make my own gui screen load? I see that in the ExtendedMenuScreen buttons like the Options Button opens up a new Screen gui that shows the options menu, but is it possible to load my own menu from a custom button? If so, how? Because these buttons in the java file load all the basic buttons, but their are already button json files for these buttons.

You'd have to add a button in code to open a gui that is not already defined in ScreenType