[KevinH]

If it is an epub3, and if the remote resource is an actual video and not a link to a web page with the video embedded, simply go to the opf manifest for this file and add "remote-resources" under properties. This tells the ereader that that specific xhtml page accesses a remote resource.

Then add a manifest entry to point to the remote resource.

Something like the following example:

<item id="content_001" href="content_001.xhtml" media-type="application/xhtml+xml" properties="remote-resources"/>

<item id="outside-resource" href="https://www.myserver.com/myvideo.mp4" media-type="video/mp4" />

But you must use a video tag in your xhtml and not an iframe as iframe's are viewed as a security issue by the epub3 spec.

If the video is small enough, just hosting it inside the epub will work just fine.