flatpak --user remote-add --authenticator-name=org.flatpak.Authenticator.Oci katello-direct oci+https://centos9-katello-devel.sajha.example.com/pulpcore_registry/

Possible paths:

Global registration.
Drop in remote config on host.

https://docs.redhat.com/en/documentation/red_hat_enterprise_linux/9/html/administering_the_system_using_the_gnome_desktop_environment/assembly_installing-applications-using-flatpak_administering-the-system-using-the-gnome-desktop-environment#proc_enabling-the-red-hat-flatpak-remote_assembly_installing-applications-using-flatpak

 /etc/flatpak/oci-auth.json

DEV Tracker for https://issues.redhat.com/browse/SAT-28556