windowed game, how to move mouse outside of game, unfocus game?

I have a mini game that is not full screen.
I want the player to be able to adjust the screen size or do other things on the computer while playing the game.
I know the person could toggle, but is there a way to make it so the player can hit a button and “lose focus” on the window to be able to move the mouse around? like a focus, lose focus option?