Using the Display object in FormExDesktop obtain the width and height of the screen your FormEx is on. Set its' width and height to the screen values and set the BorderStyle to 0. Make sure you have some way of re-setting the BorderStyle to 4 by clicking on the FormEx or by some other means.
Unless full screen is a separate FormEx being shown you will probably want to save the current position and size of the FormEx and restore those previous FormEx settings when exiting full screen.