I will use a movie I added earlier today as an example here. When you add a movie to the database (https://www.themoviedb.org/movie/359151-tanna) you can choose to quick-import the movie to letterboxd. Then you take the movie number (the TMDb ID, in this case 359151) and add it like this (without the spaces): letterboxd.com / tmdb / 359151 . That will quick-import the movie. Otherwise you will have to wait at least 24 hours for it to appear. Sometimes it can be quicker, sometimes slower. When quick-importing not all information may be added to Letterboxd - that will come with the next update.

So in order to add something to Letterboxd, you first have to add it to the database here.