How to add the jane street opam repository is introduced here.
Here, we assume the branch to add is core-branch. You may need to replace the core-branch with the branch you need.
$ git clone -b core-branch https://github.com/janestreet/opam-repository.git
$ opam repo add js opam-repository
Or, directly add the remote branch as an opam repository.
$ opam repo add js https://github.com/janestreet/opam-repository.git#core-branch