How to take over app ownership?

Hello community,

I have found and read the topic about the transfer of ownership of apps.
I am in the position that I am currently the co-maintainer of the cookbook app but the main maintainer has left the project for personal reasons in a hasty fashion. As a result, I have the private key to create/sign app releases and have access to the source code. Unfortunately, I do not have admin rights on the app in the app store. As a result, I am sort of restricted in my options.

Also, I would like to add another co-maintainer to prevent the same situation from happening again if I fail for some reason to hand over the project (or be not available for some time).

So, I think, I need contact to a human administrator as the suggested solutions so far do not cope with my case. How can I find such an admin or her/his account name?

Thank you very much!
Christian