YouTube/Vimeo videos
1. Copy the iframe embed code from YouTube/Vimeo. It can be found in their respective "share" options.
2. Create a new Iframe block in your Slides presentation.
3. Paste the copied embed code into the iframe's source input field (highlighted in the screenshot below).
Example iframe embed code from Vimeo to test with:
<iframe src="//player.vimeo.com/video/93361282" width="500" height="313" frameborder="0" webkitallowfullscreen mozallowfullscreen allowfullscreen></iframe>