In order to add a video on a slide, just link it from any of the following online platforms: Youtube, Vimeo, and Wistia.
When editing, go to Add Content Block and select Video. The element will give you the field to paste the video URL link, from the platforms mentioned above.
Keep in mind that Video can only be added to empty slides, as it will be reproduced in full screen whenever the slides are being viewed and other elements could not be seen.
After adding the video, a still image will show up in the editor and the video will be automatically reproduced whenever you hit its slide, when viewing or presenting the deck.