How to use the docusign-esign.ApiClient function in docusign-esign

var EmbeddedSending = function () {
  // TODO: Enter signer info and path to a test file
  var signerName = '[SIGNER_NAME]';
  var signerEmail = '[SIGNER_EMAIL]';

  // point to a local document for testing
  var SignTest1File = '[PATH/TO/DOCUMENT/TEST.PDF]';

  // we will generate this from the second API call we make
  var envelopeId = '';

  // initialize the api client
  var apiClient = new docusign.ApiClient();
  apiClient.setBasePath(BaseUrl);

  // create JSON formatted auth header
  var creds = '{"Username":"' + UserName + '","Password":"' + Password + '","IntegratorKey":"' + IntegratorKey + '"}';
  apiClient.addDefaultHeader('X-DocuSign-Authentication', creds);

  // assign api client to the Configuration object
  docusign.Configuration.default.setDefaultApiClient(apiClient);

  // ===============================================================================
  // Step 1:  Login() API
  // ===============================================================================
  // login call available off the AuthenticationApi
  var authApi = new docusign.AuthenticationApi();

  // login has some optional parameters we can set