I'm accessing my report through a URL:
http://reportserver/ReportServer /Folder/ReportName&rs:Command=Render&rs:Format=CSV&rc:Parameters=true

I am getting an error: This report requires a default or user-defined
value for the report parameter 'beg_dt'. To run or subscribe to this
report, you must provide a parameter value.
(rsReportParameterValueNotSet) Get Online Help

I do not want to pass the report parameters (to and from dates) through
the URL. I want the dates to be entered by the user running the
report. If I enter default parameters in Report Manager I am not prompted
for the dates.

Re: Accessing report through URL not getting prompted for parameters

Brad Syputa - MS

This will depend on what render format you want to specify in the URL. For rs:format=CSV, you will either need to have default parameters or specify in the URL. You cannot specify parameters to that format post render.

If you are using rs:format=HTML4.0 in the report viewer(report manager or report server), you do not have to specify the parameters. You should be prompted if the parameters are set to prompt with no default values. If you have default values, the report will execute.