When launching SSRS reports by URL, it is often necessary to include parameters. To do so, simply include the parameter name, an equal sign, followed by the parameter.
So param=value for a report where CustomerID is 1 would look like:
http://localhost/ReportServer/Pages/ReportViewer.aspx?%2ReportFolder%2DemoReport&rs:Command=Render&CustomerID=1
Now, let's say you want to pass in a NULL value for CustomerID. That would look like:
http://localhost/ReportServer/Pages/ReportViewer.aspx?%2ReportFolder%2DemoReport&rs:Command=Render&CustomerID:isNull=True
That didn't work for me.
You need to change the localhost to your server (and port number if used)