How do i activate a function - toggle button

Hi im trying to make a toggle full screen button but not sure what to put for the fuction thing
the script im calling it in has :
public static bool fullScreen;

and later

void Fullscreen() {
Screen.fullScreen = !Screen.fullScreen;
}

what settings etc do i have to call on the toggle button to make it call this so when they toggle the button it switches between windowed and fullscreen