With OptiMonk campaigns you can set up different button events depending on which action you wish to take place when someone clicks on the button on popup. You can find the button event if you click on the campaign, then on the variant and go to Popup section:
If you select it as Next, you activate the Thank you page which will be displayed when someone clicks on the button of the campaign.
By selecting Redirect, it allows you to add a URL and whenever someone clicks on the button, then will be redirected to that page
When it is Close that means that the button will have the functionality of closing, the popup will disappear.