Videos can be added to your slides, linked from 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. You can just copy and paste the browser's search bar
If it's only the video, it will fill the slide. You can also manually resize and reposition it if you have other elements in the slide.
Keep in mind you'll need to hit the Play button in the video for it to start playing, when presenting.