This is specifically talking about the new Chromecast.
Like 2 weeks ago, you could google a movie or show and somewhere in the search result "card" was a button to add it to your Google watchlist. As of yesterday when I tried to do it, I couldn't find the button anymore. I have tried today with different movies and shows but it just seems to be gone, unless they moved it somewhere that I haven't looked.
I've just been using that feature. Works for me pal, shows up just below the poster art for the show or movie.