In ooDialog you can connect a Windows event to a method in a ooRexx dialog class. When the Windows event happens, the method in the dialog is invoked.
Several of the internal methods related to this are allowing the empty string to be used as the method name. This of course doesn't work. In addition, in the doc the railroad track diagram for some of the methods indicates that the method name is optional, although the text itself doesn't indicate that. The method name can not be optional.