It works also with the Closed event.
Unfortunately, you don't give enough information.
What is the content of lstSystemID(0)?
Is it the list you transmit when you initialize the wheel?
You have
lblSystemName as the event name and
whlSystem as the wheel name is it the same object?
I tested it in the ClsWheel demo program like this:
Sub whlCustomSingle_Closed(Canceled As Boolean, Selection As String)
Private day As String
Private Index As Int
day = whlCustomSingle.Selection
Index = lstCustomSingle(0).IndexOf(day)
Log(day & " / " & Index)
End Sub