Getting a streamable link is done by the client.
You basically need to send the service a detailed list of the type of media you can play, and it will give you a transcode URL to call if it thinks you need transcoding in order to play this file.
If you don't and can play the file directly it won't give you a transcode URL.
POST <server_address>/Items/<item_id>/PlaybackInfo?UserId=<current_user_id>