Ed,
As near as I can tell, you're already familiar with and using the Event structure and you're just trying to get at the value of the subsequently highlighted text in your string control.
Two of the properties of the string control--accessible via a Property Node or a control reference--are Text.Selection.Start and Text.Selection.End . These define the beginning and end position of the currently-highlighted text. Using these values, you can get a String Subset of the text control that is the highlighted text only.
Hope it helps,
John