How to I make it so that my Game's Window can be resized in window mode?

I want it so that the Maximize and Minimize buttons work and you should also be able to Resize the window as well

You can set it in Edit->Project Settings, the size set will be remembered between sessions, so if you don’t want that you have to set the size at startup with Screen.width and Screen.height.

How do I do this on an already built game without access to source?