You would have had to enter your Hertz Gold Rewards member number at the time of booking on the checkout page. If you didn't do that, there's no way to enter it after the fact, so the only way to get the reservation associated with your Hertz account is to re-book with your member number and then cancel the old one. Keep in mind that rates may have changed since you originally booked, so we always recommend booking the new reservation first before canceling the old one.