I have Anmeldung in my partners/friend’s house, and I don’t have a rental contract on my name. What should I do?

In Berlin, your partner/friend can provide a written and signed document that states that you can live there.

And how much you pay for the rent. You need to add the proof of your rental payments. 

If you live there for free, it needs to be stated. You can upload this document next to the rental contract of your partner/friend.

In Potsdam you need to provide different documents for your accommodation, and you require a rental contract on your name directly from the landlord and not by friends. 

Only a married person living with its spouse can provide other documents.