Whenever the user presses a button on the mouse, an OnMouseDown event goes to the object the pointer is over. The object can then respond to the event. 

To respond to a mouse-down action, attach an event handler to the OnMouseDown event. 

The Code editor generates an empty handler for a mouse-down event on the form:

procedure TForm1.FormMouseDown(Sender: TObject; Button: TMouseButton;
Shift: TShiftState; X, Y: Integer);
begin
end;

void __fastcall TForm1::FormMouseDown(TObject *Sender, TMouseButton Button,
  TShiftState Shift, int X, int Y)
{
}