First, check whether you need to notify National beforehand for that driver at all, or if they're an automatic authorized driver. For example, with Hertz, if I'm on a corporate rental any other employee of the company is allowed to drive it. And for a personal rental under Hertz Gold, my spouse is an authorized driver, without having to designate her. In several states that's true for spouses by law for all rentals.

If not automatic, they you will likely have to go to the counter for the additional driver's license to be verified.