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.
Best Answer
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.