You want to use “Continue Watching” (“On Deck” doesn’t exist anymore) as a kind of “watch list”.
That’s currently not possible in Plex. You might want to add your vote to this thread Feature Request - Watchlist
If you add a new season of a tv show which you have been watching earlier, it can show up in “Continue Watching” automatically.
Activate “Include season premieres in Continue Watching” under
Settings - Server - library - (Show Advanced)
