How to get the "selected index" of a control ?

Hello,

I created my own page with InstallOption / ModernUI.

I'm distributing some plugin and want the user to select one installed "host"-application from a ListBox.

The problem:
If I use the NOTIFY functionality of the ListBox, the "State"-Property contains the Text from the Box - but I need the *selected Index*, because I'll have to map each Selection to a bunch of different RegKeys to write !!
(Think of the ListItems as "Labels" that each have to map to some "application data")

Any help would be greatly appreciated !

greets
Florian