procedure TForm1.Button1Click(Sender: TObject);
var
  p: TNotifyEvent;
begin
  TMethod(p).Code := MethodAddress('Button2Click');
  TMethod(p).Data := Self;
  if Assigned(p) then
    p(Sender);
end;

{или}

uses
  typinfo;

procedure TForm1.Button3Click(Sender: TObject);
var
  p: TNotifyEvent;
begin
  p := TNotifyEvent( GetMethodProp( Button2, 
                                    GetPropInfo( TypeInfo(TButton),
                                                 'OnClick')));
  if Assigned(p) then
    p(Sender);
end;