To add a video on your site so it opens in a modal window, click on Publish and then go to the Popup tab.
You have three ways to add it to your page. Video Player, Text Link, and Image.
Video player will show a standard video player image when will launch the modal when clicked.
The link will add a text link to your page and when clicked will launch the modal.
Finally you can upload or link to an image that you want to use and when clicked it will launch the modal.
Just configure which type you want and then click to copy the embed code and add it to your page: