code: function(object, object_id, callback) {
      if (typeof object_id === 'function') callback = object_id;
      request.post(this._endpoint, {
          form: this.subscribeParams(object, object_id)
        }, function(error, response, body) {
          if (!error && response.statusCode == 200) {
            var res = JSON.parse(body);
            callback(null, res.data);
          } else {
            callback(error, null);
          }
        }
      );
    }
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