A quick update - I just logged in to my Hertz account today and noticed my existing account has been upgraded to Five Star and I hadn't contacted Amex so it must be automated some how. All I did was add my Platinum card to my Hertz account.

There is a link on the Amex site for enrollment but it is only for new Hertz accounts so I didn't do it and was planning to call them but looks like I don't need to now