GI Parameter - filter the counted number after grouping

Hi, I am creating a GI and meet some problem on parameter.

Here are my parameters requirement:

1. No of PO (counted nbr, not ref nbr)

I am using COUNT at the aggregate function and grouping the record with vendor, the result of the ref num will become counting how many PO of the vendors. 

My question is how can I make the parameter to filter the counted number but not the ref number? attached with the GI XML, thank you.