Put the movie into its own subfolder, named exactly like the Movie's title plus release year in parentheses `21 Jump Street (2012)`
Make a new text file, copy this into it:
http://www.imdb.com/title/tt1232829/
rename the text file to
21 Jump Street (2012).nfo
Put this file beside your movie file.
This the link to the IMDB for this movie. The plex movie scanner looks for this file and uses the IMDB ID (tt1232829) to help matching the movie.
You can use this trick for particularly hard to match movies (usually those titles that start with a number).
