For security reasons, access to the apartment and therefore to your room is only allowed after check-in.
Additionally, in buildings where a concierge service is provided, once you have taken possession of your room, you will need to fill out a proxy form for the mail collection. You can sign the proxy at the concierge's front desk after you have checked in. You will only be able to receive your mail once you have given your signed proxy to your concierge.
