Is there any way to keep the game window running when it's not focused?

I've played games where I can just add a line of code myself to keep the window running while in the background. There's been a lot of times I alt-tab to do something, but still want the game to keep going. I would like to know if that could be implemented into the game(LonaRPG), or if someone could tell me what line of code, and where I should put it.