register_new_matrix_user -k 's3cr3t' -u dave -p 'secret' --no-admin http://localhost:8008

Uses -k/--shared-secret to pass the registration_shared_secret inline instead of reading it from a config file, with the homeserver URL as the final argument. The secret must match the registration_shared_secret value in homeserver.yaml or registration fails. Prefer -c with the config file so the secret is not typed in the open. Works against a remote server if the admin API is reachable.