I regret to say that, while our apps (Apple & Android) supports adding items to a Favorites List, we do not yet support that on the Web. Adding this feature to our website is something that is on our update list to make available to our users.
If you have a Roku, Apple TV 4th gen, or Amazon Fire TV, you can download our apps in those third-party App Stores. They have the "Watch List" feature as well.