Link a subtitle to an available video. Don't forget to use POST as the request method.
https://streamio.com/api/v1/videos/video_id/subtitles.format
POST
201
Pretending we have a video with id 4cea88e454129010f2000018 and we wish to add a subtitle with the id 4cd2be8454129008dc00000e.
curl -u username:password \
-F subtitle_id=4cd2be8454129008dc00000e \
https://streamio.com/api/v1/videos/4cea88e454129010f2000018/subtitles.json