Go to the website of the broadcast. There will be a link for beginning the broadcast, usually labeled "Listen now", or "Tune in now". Hold the control key while mouse clicking on this link, and select Copy Link in the contextual menu. Paste (command v) that link into the Processing field in the Processing pane of the WireTap Studio Session.
For more information on obtaining the URL from a RealPlayer, iTunes or Media Player broadcast launch WireTap Studio and select WireTap Studio Help from the Help menu. Open the Index and see the section called "Setting the Pre-Processor Options for a Recording Session".