I want a button to activate a pop up in popup pro

Hi, I am trying to use PopUp Pro to display a sign-up form and I want the form to display when I button is pushed.

I have the pop-up built with an image and form inside pretty much the way that I want it but instead of popping up when a button is pushed it pops up on page load.

How do I make the popup open when a button is pushed?

Thanks,