In page editor select source code mode.

Then add the video link embedding code. For example, on YouTube (on a computer not mobile) when you click share or as a separate option for a video there is an “embed” option that gives your a short snippet of HTML that has an IFRame tag and a link to the video. Copy that snippet and paste it into a <div> tag in “source view” in the editor. It’s the button that looks like this <> when you edit your page.

<div><iframe width="100%" height="315" src="https://www.youtube.com/embed/dM3KSmvQmkU" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe></div>