type

 TForm1 = class(TForm)

 ...

 protected

  procedure CreateParams(var Params: TCreateParams); override;

 end;

var

 Form1: TForm1;

implementation

{$R *.dfm}

procedure TForm1.CreateParams(var Params: TCreateParams);

begin

 inherited;

 Params.Style := (Params.Style or WS_THICKFRAME);

end;