Use result of listbox1 in runtime calculation in listbox2

I want to use the pick results from listbox1 in a runtime calculation of listbox2.  how do i access the result of listbox1 so i can use it in a runtime modification of listbox2?

example:

a user selects an listitem of county names in listbox1

in listbox 2 i want to calculate the lable for listbox 2 with the selected listitem from listbox1

J