In your .web project, with your Silverlight application, right-click and select Properties.
In the section "Web" there is a section "Action". In the "Start Action" section, it will probably be set to "Specific Page" and display your initial hosting page. e.g. Default.aspx
Just change the url there to add options like Default.aspx? Test = 123
(Of course, an alternative is to simply change your browser URL manually as soon as the application starts).
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