type
  TPopupBox_helper = class Helper for TPopupBox
    procedure ShowPopup;
  end;

procedure TForm2.FormCreate(Sender: TObject);
begin
   PopupBox1.Items.Add('Item1');
   PopupBox1.Items.Add('Item2');
   PopupBox1.Items.Add('Item3');
   PopupBox1.Items.Add('Item4');
end;

procedure TForm2.Button1Click(Sender: TObject);
begin
   PopupBox1.ShowPopup;
end;

{ TpopupBox_helper }

procedure TPopupBox_helper.ShowPopup;
begin
   Self.Click;
end;