In the General tab of your popup, the last section is 'BUTTON ACTION'.
If you deactivate it, you will not get any call to action in your popup.
If you activate it, you will get a call to action (CTA) just after the text.
Write the text ('click here!') and add the URL that will be opened in a new tab when a viewer will click on this CTA. You can use a URL shortner such as https://goo.gl/ if you want to track the CTA analytics.
In the Design tab, there is a 'BUTTON ACTION' section. From there, you can change the following parameters, specific to your CTA:
- size & position compared to the text
- text color with and without mouse over
- background color & opacity with and without mouse over
- border size, color, opacity & radius with and without mouse over
- font size