Selecting an Expression in a List Box using a Button

[#1]

Hi All

I am trying to make a button the opens a chart and selects a couple of list boxes.  The problem lies as one of my list boxes is generated by the following formula

=if(if(Len(Keepchar([Policy No],'X'))=1,Aggr(count([Claim Number]),[Policy No]))>1,'Multiple')

How do I select this as there is no specific field to select.