code: function(options, callback) {
      return AuthenticationRequest.builder()
        .withPath('/api/token')
        .withBodyParameters({
          grant_type: 'client_credentials'
        })
        .withBodyParameters(options)
        .withHeaders({
          Authorization:
            'Basic ' +
            new Buffer(
              this.getClientId() + ':' + this.getClientSecret()
            ).toString('base64')
        })
        .build()
        .execute(HttpManager.post, callback);
    }
docs: Request an access token using the Client Credentials flow.
  Requires that client ID and client secret has been set previous to the call.
  @param {Object} options Options.
  @param {requestCallback} [callback] Optional callback method to be called instead of the promise.
  @returns {Promise|undefined} A promise that if successful, resolves into an object containing the access token,
  token type and time to expiration. If rejected, it contains an error object. Not returned if a callback is given.