How to set the screen brighness keys to affect the external monitor? I mean instead of affecting the brightness of the built in display of a Macbook.
Unless it's an external Apple monitor or built-in screen (i.e. on MacBook or iMac), the brightness of the monitor itself cannot be controlled from the keyboard. There is software available to control what the computer is outputting, therefore adjusting the brightness before it ever reaches the monitor, but there is no way to adjust settings on the monitor itself.