Fortnite normally opens up on the monitor in which the icon was clicked to open the game up. So the first thing is to make sure that the fortnite launcher is opened on the monitor you want the game to load up on.
And then click launch. If this does not work and it loads it up on the wrong monitor. Then there is a simple keyboard shortcut.
When the game has loaded, press Windows + Left Shift + (Left or Right arrow key) depending on which way you want to send the application.
For example if Fortnite is on your left monitor and you want it on the right one, you’d press Windows + Left Shift + Right.
Likewise if it’s on the right monitor and you want it on the left monitor. Press Windows + Left Shift + Left arrow key.
Although in one of the later versions of Fortnite is was fixed to load up on whichever monitor you opened the application up on.
This shortcut is useful and can be used on any application you have on your PC. Useful if you want to switch the screen that your browser is opened up on.