How to get to the code on someone else's self-written admin panel?

Question

Domain Name System

I only have a few business card sites on my account, so I'm an amateur.
But here it took only a little to alter the already finished site. They gave the admin panel, it turned out that the site engine was self-written, there is no editor or open source. They gave me a username and password for the domain, but where do I enter them? The search for the resource where this domain was registered was unsuccessful, and the creators of the site are not announced. How to get to code editing?

Answer 1

probably this is ssh access or ftp at worst