I think you just got a bad rep. I recently (6 months ago?) finally added my contract to my account after way too many years of typing it in manually every time.

I simply called the US 1-800 number and the rep had it added in seconds after the pleasantries were exchanged. I was rather astonished at how easy and simple it was. They treated it like a Tier 1, do-it-everyday task. I would call your geographically-correct customer service number. (Basically HUCA, but from email to phone.)

ETA: This is one case where Hertz has EHI beat. I like that you can associate all your contracts and discounts onto your Hertz profile, then pick-and-choose the one you want during the reservation. I don't know why EHI has left this 'unfixed' for however many years it has been this way. (10+, maybe? Quite a while, however long.)