buildAuthorizationWithFields method 
    
    
    
  Implementation
  Credential? buildAuthorizationWithFields(Map<String, String?>? parameters) {
  if (authorizationFields == null ||
      authorizationFields!.isEmpty ||
      parameters == null) {
    return null;
  }
  var fieldUser = authorizationFields![0];
  var fieldPass =
      (authorizationFields!.length > 1 ? authorizationFields![1] : null) ??
          'password';
  var user = parameters[fieldUser];
  var pass = parameters[fieldPass];
  if (user != null && pass != null) {
    return BasicCredential(user, pass);
  }
  return null;
}