forum.freeciv.org

If you just want to give rights to a user that already connected, you can use cmdlevel to set the privileges of a) a single connection, b) the next person to connect, c) all future connections or d) all connections (present and future).
If you want more persistent privileges, you'll have to use an authentication database. Details on how that works are in /doc/README.fcdb .

That said, when connecting a client to localhost, your connection should typically receive cmdlevel "hack".

Yup, just realized that now. I didn't actually work with fcdb, ever. That's not a feature that exists at the moment. At the end of the readme is linked hrm Feature #657143, which is about that.