docs: function addPsidtoCustomLabel (psid, label_id) {
    return new Promise (async (resolve, reject) => {
      if (!psid || !label_id) {
        reject('PSID and label_id required');
        return;
      }
      let options = {
        'path': `/${label_id}/label`,
        'payload': {'user': psid}
      };
      try {
        let response = await  this.callCustomLabelsApi(options);
        resolve(response);
      } catch (e) {
        reject(e);
      }
    });
  }