How do I make a popup display when a button is clicked?

Hi,

On my website, I have a "call to action" button that says "SIGN UP to hear more". When a visitor clicks on that button, I want to display a popup that has a Mailchimp subscription form on it.

I've created the popup using PopUp Pro plugin, however the settings in the following sections don't mention "on button click":
a) When to show the PopUp
b) Displaying Conditions

Is there an easy way to make a popup display when a button is clicked?

Many thanks,

Rod