Wix just released the ability to add popups to any page on your Wix Website. This page shows you an example of how it works. You have full control over the design of the popup, and can add subscribe features, images, event info, etc. The new feature is called Lightboxes and can be found under the Wix + tool, called "Lightbox". 

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My only caution: popups annoy people. Well, at least they annoy me. So if you're going to do it, make it worthwhile to your user!