If the first part of the subtitlename equals the moviename and the subtitlefile is
either in the same map/directory as the movie
or in one of the subtitle directories which you can define under .. Preferences > Subtitles (south-east quarter) it will indeed play automatically unless you've "Subtitles disabled" under Preferences.
Further there is a possibility there to "Auto load any subtitle in current directory" (i.e. irrespective of its filename).
Another option is to use an ini or bsi file to start (read item
(8) of my Help2Help-post).