There are several ways to invoke WebFile. You may use a standard HTML form like that below, or you can put all the parameters on the invocation and tie it to a button or even use a straightforward HREF link like those shown below. This page shows all three combinations, but you'll want to use only one style for your application. Save this HTML page and look at the source to see how WebFile is being invoked.
These links will use this window, so you'll want to specify the keyword 'url' in the configuration file.