It's not a windowclass exactly. The base FG application code knows to handle "url" differently - that it won't be a windowclass. All it does is launches the default O/S browser with the value of the second argument (URL string). Then it continues on its merry way, not knowing or caring if the browser opened and displayed the URL.
So, if you want to try to do something in the browser, you'll need to code something into the resulting resource pointed to by the URL - if it's an HTML page, put some code in to do what it needs to do then close the browser, for example.