You should put there path to your script (on your website) and specify which info you like to transfer.
For example, it could be "http://www.your-site.com/script.php?artist=%artist&title=%title". This will call "script.php" with two parameters: "artist" - contains artist name, and "title" - song title.
You can also add any custom parameters, like "&from=radioboss" so your script will know where the data comes from.
If it shows you 404 error, this means, you incorrectly entered script address.