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<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: