Specifying group selection formula via URL

[medeya]

I have a report , that groups records by one field ( {Table_Name.Agent_Name}). I am trying to create a web report that would allow specify the GROUP SELECTION formula via URL. I need to get the top 5 'Agent_Name' based on Sum({Table_Name.Total_Charges} field. I tried doing this: http++++++++++++.rpt?gf=NthLargest(5,{Table_Name.Agent_Name}), but it doesn't work.
Could someone tell if it is possible to do it via Url at all or should I go from having a parameter field in the report and using it to change the group selection formula dynamicaly ?

Thanks !

Medeya.