The Standalone Report Designer is a desktop application. When you preview reports in it, they are processed locally in the context of the desktop application.
Once the report is designed and included in a web application, you can get the Session variable and use it as a value for the report parameter in code e.g.:
var URS =
URS.Uri = Server.MapPath(
//specify values to be passed to report parameters
//the ReportSource.Parameters collection is mapped to the report's ReportParameters collection by Name
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