Multiple monitors - How do you make UE4 play on a particular monitor?

There’s a trick to this because it was working fine until I moved the window to the other screen. Now it won’t run by default on my other monitor and I can’t remember how I managed to make it work in the first place.

Does anybody know how I can get the game previews to stick to the other monitor again?:

Screen 1 = Engine
Screen 2 = Game Window/Standalone Game



Can someone answer this question? What is the command line arg or game setting that needs to be set for this?