Doesn't control brightness of connected external monitor

[BrianGilbert]

I have a Ultrafine5k and when I use the brightness dragger from the menu bar on the ultrafine screen it changes the broghtness of my MBP screen rather than the ultrafine monitor.

If you figure out a way to get this working I'd love a way to control the brightness of both screens with single slider as a preferance.

[jorisw]

FWIW, it does work for my external monitor, of the MacBook Pro is closed.