Hello,
The shortlist app is not currently set up to support video. If you have some javascript skills, the source code could be modified to achieve embedding a video. Alternatively, I would encourage you to take a look at two other storymap templates that do support video, map tour (Map Tour App | Story Maps) or the new map journal (Map Journal App | Story Maps).
You can embed a video in the Map Tour template, though, as I show in this video:
Esri Storymaps for Education: Embedding Video - YouTube
For the shortlist, you could insert a LINK to your video but not at the current time embed a video into it.
Joseph Kerski