When I debug a Silverlight app, Visual Studio will just open the page with no extra URL parameters.
So, the url will look something like this: http://localhost:65351
On your .web project, hosting the Silverlight app, right click and select "Properties".
Under "Web" there is a "Start action" section. Under "Start action" it will probably be set to "Specific page" and shows your start hosting page. e.g. Default.aspx
Just change the URL there to add your parameters e.g. Default.aspx?test=123
(Of course the alternative is to just change the URL of your browser by hand, once the app has started).