https://www.hilton.com/en/offers/car-rental/ , then click on existing reservations maybe? T&C says it has to be booked on this page.

Qualifying Rentals are defined as follows:
The Primary Driver's Hilton Honors number must be provided at time of reservation or rental. The name of the member associated with the Hilton Honors number provided must match the Primary Driver's name on the rental to qualify for points awards. Only one Hilton Honors member per car rental will be credited with points. Special contracted corporate or promotional rates such as employee, net tour, travel agent, group, wholesale or package rates, crew, insurance/dealer replacement or other programs that prohibit the issuance of rewards or non-revenue rentals do not qualify for points.