[Closed] dotnet listbox selection handler

How do I create a handler for dotnet list box so I can get a button’s text to reflect a selection change?
Like mxs:
on listbox selected index do ();
should the equivalent be:
on dnlistbox selection arg do ()?
or on dnlistbox focus arg do ()?
Or do I have to create a seperate eventhandler for the object?
SOrry for the newbie type question…