How do I enter my myGermany Address in vendors checkout?

The billing address should be your own address.
The delivery address is your myGermany address:

YOUR NAME, myGermany GmbH (first and last name, Vor uns Zuname)
Nordstraße 5 (street, Straße)
99425 (ZIP, Postleitzahl)
Weimar (city, Stadt)
Thuringia (district, Bundesland)

Phone:
(international format) +49 (0)3643-8780131
(national format) 03643-8780131

Please also check here.