Get the index of listbox

Hi,

I have one screen that has one field with f4 help(dictionary).

There are two fields in input help.

eg. Resource Plant

-

Rsc1 Plt1

Rsc1 Plt2

In above case if I select second value i.e. Rsc1 then I want Plt2 to be selected.

In code I am using selected value of resource and fetching Plant value from database.In this scenario I want Plt2 to be returned by select query.

Is there any way to know which value from the help has been selevted by user?

Plz help.

Thanks.