A couple of years ago this post raises the question of an Enterprise (Portal) Cascade Storymap not being able to add a YouTube video.
Owen Evans responded with: 'We are looking at supporting this in 10.6.1 (or possibly a 10.6 patch). When this is implemented an administrator will need to add your own YouTube API key to a configuration file in the Cascade app on your portal.'
I'm still getting the same problem today and we are at ArcGIS Enterprise 10.6.1: has this been implemented yet and if so, where is the documentation describing the procedure?
That should just about do it....