code: function TopUp (apiKey) {
    if (!(this instanceof TopUp)) return new TopUp(apiKey)
  
    if (!apiKey || typeof apiKey !== 'string') {
      throw new SMSGHError('TopUp expects an api key as a string')
    }
  
    this.apiKey = apiKey
  }
docs: TopUp description
  @param {String} apiKey api key provided by SMSGH