How to add a lightbox?

You can only have lightboxes in the Pro version of Smart Slider. There are two ways you can create lightboxes.

At your slider's settings -> Slides tab, you can turn on the Backgrounds in lightbox option.

This will create lightboxes from your Slide Background images.

At any link option, like the link of a button or the slide

within the Lightbox tab you can insert image urls or any other urls.