Try going to
https://www.openmailbox.org/index and clicking on "Register Now" at the bottom, or go directly to
https://www.openmailbox.org/#register and try from there.
I didn't take the final step toward creating a new account because I don't need another one, but there was nothing on either page to indicate that registrations are closed.
If you want one, try creating an account and let us know if you succeed!