You could use the command line client here: https://github.com/uroni/urbackup-server-python-web-api-wrapper or directly add it to backup_server.db with the sqlite shell.
import urbackup_api
server = urbackup_api.urbackup_server("http://127.0.0.1:55414/x", "admin", "foo")
server.add_extra_client("1.1.1.1")