One point of clarification—the rental does not need to be booked directly with Hertz, but the system you book through does need to support the application of Hertz Gold member numbers.

For example, when booking through an OTA like Priceline or Expedia, there is a place to enter it on the checkout page. Most corporate booking tools like Concur also have the ability.

In short, your Hertz number can’t be added after the booking is made, but I would be shocked if your corporate booking tool doesn’t support this at the time of booking. If it doesn’t when something is very wrong with the system or implementation.