This global scope, built-in variable can be used to access the application surface using any of the surface functions. This surface is permanently available and is where the bulk of drawing is done by GameMaker Studio 2.
application_surface;
Pointer
surface_resize(application_surface, display_get_gui_width(), display_get_gui_height())
The above code will resize the application surface to have a 1:1 ratio with the GUI layer.