A string value specifies the name of the item. The string can contain wildcard characters. You can also specify the index of the item as a string, in the form "[n]", where n is the numerical index of the item.
listbox.select(0) ' Selects the first item, which has the index 0.
listbox.select("[0]") ' Also selects the first item.
listbox.select("apple") ' Selects the first item with text "apple".
listbox.select("apple[1]") 'Selects the second item with the text "apple".