code: 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);
      }
    });
  }
docs: Associates a user's PSID to a custom label.
  @param  {Integer}  psid  PSID of the user to associate with the custom label.
  @param  {Integer}  label_id  The ID of a custom label. Created with {@link #createcustomlabel|createCustomLabel()}.
  @return {Promise<Object>}  The API response
  @memberof  Client#
  @example
  let psid = 49670354734069743,
  custom_label_id = 0957209720496743;
  Client.addPsidtoCustomLabel(psid, custom_label_id)
  .then(res => {
  console.log(res); // {"success": true}
  });