I've booked a prepaid car 2 weeks ago with Hertz but I forgot to add the age waiver PC# 144314. When I try to modify my reservation an error pops up because apperently it's not allowed to add a PC code after the reservation is made, which I can understand. How can I solve this?

I've added the coupon code in the 'Customer Remarks' section: "YOUNG DRIVERS FEE WAIVER PC 144314". Would that work? Or will the clerk just ignore that and charge me because of my age?