Create a form and add the following code:
type
TFrmBase = class(TForm)
protected
procedure CreateParams(var Para: TCreateParams); override;
{....}
end;
{.....}
implementation
procedure TFrmBase.CreateParams(var Para: TCreateParams);
begin
inherited;
Para.WndParent := GetActiveWindow;
end;
Now create your modal form inheriting this form. Create it somewhere and call its ShowModal method. Now it will flash if you clicks in the parent form.
Taken from Torry's Delphi Pages