At work I use a Lenovo X220 and the lenovo docking station, with a Samsung monitor, and USB mouse & keyboard.
When I dock the laptop I then need to press [Fn] + [F7] until the display comes up on the external monitor.
My USB keyboard doesn’t have a [Fn] key, but I found this post which said [Win] + [P] may help under Windows.
I tried it under Ubuntu (16.04) and it gives the same functionality as pressing the [Fn]+[F7] key.
Saves me having to stretch across to the laptop now.