type
  TForm1 = class(TForm)
  private
    { Private-Deklarationen }
    procedure WMSYSCOMMAND(var Msg: TMessage); Message WM_SYSCOMMAND;
  public
    { Public-Deklarationen }
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

procedure TForm1.WMSYSCOMMAND(var Msg: TMessage);
begin
  { WM_SYSCOMMAND abfangen }
  if Msg.Msg = WM_SYSCOMMAND then
  begin
    { wParam prüfen }
    case Msg.wParam of
      SC_MINIMIZE:
      begin
        ShowMessage('Hello world');
        Inherited;
      end;
      SC_CLOSE: Close;
    else
      { alle anderen Fälle weiterreichen an die standard Fensterprozedur }
      Inherited;
    end;
  end;
end;

procedure TFrmMain.Handler(Sender: TObject);
begin
  // Tue irgendwas beim minimieren
end;

procedure TFrmMain.FrmMainCreate(Sender: TObject);
begin
  Application.OnMinimize := Handler;
end;