If you have a execute your class after your command , paste the command dodefault() in your event.
Dodefault() - > Execute a cod of class
Nododefault() -> Not Execute

FYI, It must be NODEFAULT without parenthesis... if you code with parenthesis in VFP8 you'll get an Error (not in previous version).

oC = CREATEOBJECT("MyLegacy")
OC.ShowMe()
DEFINE CLASS myLegacy as myCustom
  PROCEDURE ShowMe
     NODEFAULT() &&Later, use it without parenthesis...
     MESSAGEBOX("New Hola World")
  ENDPROC
ENDDEF

DEFINE CLASS myCustom as Custom
   PROCEDURE ShowMe
      MESSAGEBOX("Hello World")
   ENDPROC
ENDDEFINE