procedure TForm1.Button1Click(Sender: TObject);
var
 rgn: HRGN;
 r: TRect;
begin
 r := memo1.ClientRect;
 rgn := CreateRoundRectRgn(r.Left, r.top, r.right, r.bottom, 20, 20);
 memo1.BorderStyle := bsNone;
 memo1.Perform(EM_GETRECT, 0, lparam(@r));
 InflateRect(r, -5, -5);
 memo1.Perform(EM_SETRECTNP, 0, lparam(@r));
 SetWindowRgn(memo1.Handle, rgn, true);
end;