FYI, your manual setting is not guaranteed to be persistent. The Form's Handle CAN be recreated dynamically during the Form's s lifetime, which would lose your manual setting. If you want to customize the Form's owner window, you need to override the Form's CreateParams() method so your manual value takes affect on every HWND that the Form creates, eg:
type
TMyContestForm = class(TForm)
...
protected
procedure CreateParams(var params: TCreateParams); override;
...
end;
procedure TMyContestForm.CreateParams(var params: TCreateParams);
begin
inherited CreateParams(params);
params.WndParent := 0; // or GetDesktopWindow()
end;