Form in a lightbox

Hi, can anyone please tell me or give an example of how the form was added to the lightbox in the screenshot page of this plugin? What is the syntax?

http://s.w.org/plugins/lightbox-plus/screenshot-4.jpg?r=907050

There’s nothing about it in the 18 pages of support. Also no mention of it in the tutorial.

https://wordpress.org/plugins/lightbox-plus/