having a dll show a form

[C to the UNIT]

Hey guys, so I am using VC++ and I am making a dll, it's an addon for a game I play (they allow custom .dll's to customize the game, you can only play it in singleplayer) and I want the dll to make a form open, so I made a new dll, made the form how I like, now what code do I use to make it so that the form will appear in game? I don't understand how to make it show, thanks in advance.