Code:

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;