I created a web map using a hosted feature service that includes an <iframe> link to a video as a field in the attribute table. It displays the linked video in a pop-up just fine when viewing the web map. When I embed the map in a storymap, however, it's just a blank pop-up. No issue with linked images in pop-ups, it just seems to be an issue with the iframe code. I'm using the approach (for the web map) described here: How To: Embed a video in pop-ups in ArcGIS Online for a hosted feature service.