To access properties/options/preferences of
any filter/codec used by BS.Player for playback of a movie-file:
[with a movie-file loaded...] (rightclick >) Options > Filters > Advanced
Then e.g. for ffdshow click on the [+] sign in front of ffdshow Video Decoder
Then click on ffdshow. This reveals several options (such as i.a. "Subtitles").
Similar for any other filters/codecs.