I found a thread from 2013 where this was not possible but I have to assume it's been rectified 9 years later.

Normally not a Hertz renter so never thought to create an account before this trip. They sent an email shilling it. Signed up, don't see anywhere to add my reservation. Spoke to them via Twitter DM and they provided instructions, but it's <24 hours before the reservation and there's nowhere to Check In on the app.

Anyone have some advice here? Five hour flight with a nine month old, really don't want to get stuck in line if it can be helped...