Simply add your Plex address to add videos by email, into the "To address" Box for the email recipient, and every time a full movie on YouTube is posted, the link will be added to your Plex Watch Later Feed!
To find your address, open Plex, navigate down to the "Watch Later" section, click the question mark and on the following page will be your custom address.
Alternatively try this URL: https://app.plex.tv/web/app#!/playlist/queue/help
Enjoy.
