Hi, https://www.mediawiki.org/wiki/Gerrit/New_repositories is the usual way for this (and using Gerrit for code review). The repository would be mirrored to Phabricator Diffusion. I am not sure if we still offer using Phabricator's Differential for code review instead after T119908 got declined.
I am declining due to the fact that of the implementation of the gitiles
plugin for gerrit which provides code viewing capabilities, and also
because of the rejection of several tasks regarding Diffusion migration and
usage as code mirror. IIRC the usage of Diffusion was being questioned
after that and/or phase-out was being planned. Feel free to reopen if in
mistake. You are however entitled indeed to request the creation of a
repository in gerrit and any Gerrit administrator will set one for you.