I emailed firstname.lastname@example.org
with a screenshot as proof of my status with Hertz and was upgraded within a couple days. However, there is a registration form that I wasnt aware of or maybe didn't exist at the time I matched... Their website states:
"SIXT will match the status of your credit card or loyalty standings with the airlines, hotel, or other rental companies*. All we would require is a document or verification screenshot that would show your most current standing with the applicable partner or company and an active SIXT account. To match your existing status send your request via the form below and we will contact you within 2 weeks about your SIXT Loyalty Status.
SIXT Status Match Registration Form Link: sixt.getfeedback.com/r/4HV0weIN/
*Here are some examples of the loyalty status match with SIXT. You can switch from Hertz Gold, Europcar Privilege Club, Avis Preferred, and more to SIXT Gold. If you have Hertz Five Star & President, Europcar Privilege Executive, Europcar Privilege Elite, Europcar Privilege Elite VIP, Avis President's Club status or equivalent, you will invited you to join the SIXT Platinum tier."