To send the currently playing song to TuneIn you first require the Now Playing plugin.
Once installed, perform the following steps in the plugin:
Under the HTTP Web Request tab:
1. Tick Send now playing track via an HTTP web request
2. Select GET method
3. Enter http://air.radiotime.com/Playing.ashx?partnerId=<partnerId>&partnerKey=<key>&id=<stationid>&title={{title}}&artist={{artist}} into the URL field, replacing <partnerId>, <key>, <stationid> with those supplied to you by your streaming provider.
4. Click Apply
Once the settings have been applied and a new valid track starts playing the message 200 OK will be displayed at the bottom of this window.
You can find more information on the TuneIn API page: http://tunein.com/broadcasters/api/