def METHOD_NAME(self, idempotency_key=None, **params):
    return self.resource._request(
        "post",
        "/v1/test_helpers/treasury/outbound_payments/{id}/fail".format(
            id=util.sanitize_id(self.resource.get("id"))
        ),
        idempotency_key=idempotency_key,
        params=params,
    )