tablet pro

Hey! I just got Tablet Pro, and have been screwing around with a custom layout, and was wondering if there was any way to make use of the Fn key while using the various function keys (F1-12). I want to set up Tablet Pro to be able to alter the brightness of my Surface Pro 6’s screen with a couple of buttons (one for brighter, one for darker), but I can’t seem to find any option for pressing the Fn key while inputting the function key press I’d need to do this. Any help would be much appreciated. Thanks!