def add_webhook(self, scaling_group, policy, name, metadata=None):
        """
        Adds a webhook to the specified policy.
        """
        uri = "/%s/%s/policies/%s/webhooks" % (self.uri_base,
                utils.get_id(scaling_group), utils.get_id(policy))
        body = self._create_webhook_body(name, metadata=metadata)
        # "body" needs to be a list
        body = [body]
        resp, resp_body = self.api.method_post(uri, body=body)
        data = resp_body.get("webhooks")[0]
        return AutoScaleWebhook(self, data, policy, scaling_group)