The system typically expects to find this file at ~$HOME/.mbsyncrc~, but you may also specify a custom path if launching the command using arguments. I chose to symlink the default location to my repository.
#+begin_src emacs-lisp
(dotfiles/symlink "~/.emacs.d/config/mbsyncrc"
"~/.mbsyncrc")
#+end_src
This repository also contains my personal encrypted auth info.