If you are linking to a URL, the Path is URL that accepts parameters in value pairs; for example, http://images.google.com/images. The "q" parameter is accepted by the URL; “q” stands for “query” and anything past in this parameter is treated as if it had been typed into a query box.
If you add the "q" parameter under Custom and map it to a report field, [Value], Report Designer will generate the following parameterized URL, http://images.google.com/images?q={Value}: http://images.google.com/images?q=Rain.