Originally Posted by AutoSlash

Book while logged out. Then call and have them add your number. You will get a new confirmation number. (Behind the scenes, the Hertz agent will cancel your old reservation and book a new one for you but will manually adjust the rate to match the old reservation.)

You can quickly see whether a reservation has a Gold number applied by logging out of Hertz.com, pulling up your reservation by confirmation number, and then clicking the "Modify" button. If it pops up an error that you need to log in before modifying, then your Gold number is successfully applied.