To send the currently playing song to TuneIn you first require the Now Playing plugin.
You need to enter: http://air.radiotime.com/Playing.ashx?partnerId=<partnerId>&partnerKey=<key>&id=<stationid>&title={{title}}&artist={{artist}} into the plugin under the HTTP tab, replacing <partnerId>, <key>, <stationid> with those supplied to you by your streaming provider.
You can find more information on the TuneIn API page: http://tunein.com/broadcasters/api/