About Usage Of Lexical Parameter in Report

Hi,
Actually I am developing a report about "Purchase Order Register" in Oracle Forms & Reports 10g.In this report I want to use lexical parameter in a function which in run time will ask the user to select a supplier name from the list of values.if the user selects any supplier then it will show it's corrseponding all purchaseorders otherwise it will show all suppliers whole purchaseorders.
In this regard I have also written a function as below:-

function Parameter_FunctionFormula return Char is
vpo varchar2(1000) := ' ';

begin
if :VENDORNAME is not null then
vpo:='and asp.vendor_name = '|| :VENDORNAME;
end if;

return(vpo);

end;

Please help me to solve this problem as soon as possible.

Thanks & Regards
Shubhojit Kundu