How to get back the missing monitor option in High Sierra's menubar? When plugging in an external monitor on my MacBook-pro Sierra I could set the monitor options to show up on the menu bar. That works like a charm. 

Since the upgrade to High Sierra, I am not able to get this option again.

I connect an HDMI monitor via Apple Adapter to USB-C and I am unable to find a way to enable this option.

Other colleagues don't have this issue with exactly the same setup.
Open System Preferences → Displays and enable ‘Show mirroring options in the menu bar when available’.

This should add a menu item  for the AirPlay menu even without an external display connected, and when an external display is connected you can use this to switch between mirror and extend.