I'll keep it short and concise so, just use Dual Monitor tool you can get from here:
https://dualmonitortool.sourceforge.net

Now install, open the app (should be in your tray), then go to Cursor > Sticky > Lock cursor onto screen, and finally, set the shortcut you desire.


Press the hotkey then open the game or open first; whichever order, it doesn't matter. Press the hotkey again to release the lock so you are able to move the cursor to your other monitor(s) again.

PS: The app is open source so it should be safe and reckon a lot of people are using it already.