code: function startBroadcastReachEstimation (custom_label_id) {
    return new Promise (async (resolve, reject) => {
      let options = {
        'custom_label_id': custom_label_id || true
      };    
      try {
        let response = await this.callBroadcastApi(options);
        resolve(response);
      } catch (e) {
        reject(e);
      }
    });
  }
docs: Start a reach estimation for the number of people that will be
  reached by a broadcast to all users or to users associated with
  a custom label.
  @param  {Integer}  custom_label_id   _Optional._ The ID of a custom label targeted by the broadcast. Created by calling {@link #createcustomlabel|Client.createCustomLabel()}.
  @return {Promise<Object>}  The API Response
  @memberof  Client#
  @example
  let custom_label_id = 3467390467035645 //optional
  Client.startBroadcastReachEstimation(custom_label_id)
  .then(res => {
  console.log(res); // {"reach_estimation_id": "9485676932424"}
  });