SSRS URL With Parameter

I'm trying to put a customer number parameter into a URL so the user does not need to select it from a drop down dialog box.

I thought the syntax for doing this was something like:

http://prmsql5/ReportServer/Pages/ReportViewer.aspx?%2fPRM%2fWennsoft+Service%2fAlt+Bill+To+Customer&rs:Command=Render&Custnmbr=00249&rc:Parameters=false&rc:Toolbar=false

But that isn't working.

Any thoughts ?

John