constructor TCustomForm.Create(AOwner: TComponent);
begin
    ............
    InitializeNewForm;
    ............
end;

procedure TCustomForm.InitializeNewForm;
begin
  ............
  Screen.AddForm(Self);
  ............
end;

procedure TScreen.AddForm(AForm: TCustomForm);
begin
  FCustomForms.Add(AForm);
  if AForm is TForm then
  begin
    FForms.Add(AForm);
    Application.UpdateVisible;
  end;
end;

procedure TApplication.UpdateVisible;

  procedure SetVisible(Value: Boolean);
  const
    ShowFlags: array[Boolean] of Word = (
      SWP_NOSIZE + SWP_NOMOVE + SWP_NOZORDER + SWP_NOACTIVATE + SWP_HIDEWINDOW,
      SWP_NOSIZE + SWP_NOMOVE + SWP_NOZORDER + SWP_NOACTIVATE + SWP_SHOWWINDOW);
  begin
    // Dont auto-update visibility if somebody else has hidden app window
    if (IsWindowVisible(Handle) = AppVisible) and (AppVisible <> Value) then
    begin
      if MainFormOnTaskBar and not AppVisible then
        SetWindowLong(Handle, GWL_EXSTYLE, GetWindowLong(Handle, GWL_EXSTYLE) or WS_EX_NOACTIVATE);
      SetWindowPos(Handle, 0, 0, 0, 0, 0, ShowFlags[Value]);
      AppVisible := Value;
    end;
  end;

var
  I: Integer;
  Form: TForm;
begin
  if Handle <> 0 then
  begin
    for I := 0 to Screen.FormCount - 1 do
    begin
      Form := Screen.Forms[I];
      if Form.Visible and ((Form.ParentWindow = 0) or not Form.HandleAllocated or
        not IsChild(Form.Handle, Form.ParentWindow)) then
      begin
        SetVisible(True);
        Exit;
      end;
    end;
    SetVisible(False);
  end;
end;

program TestIcon;

uses
  Vcl.Forms,
  Unit1 in 'Unit1.pas' {Form1},
  Unit2 in 'Unit2.pas' {Form2};

{$R *.res}

begin
  Application.Initialize;
  Application.CreateForm(TForm1, Form1);
  Application.Run;
end.