property Authentication: TIdAuthentication;

Authentication is a TIdAuthentication property that represents the object used to perform HTTP Access Authentication. Authentication will normally contain an instance of a TIdAuthentication descendant, like TIdBasicAuthentication or TIdDigestAuthentication. 

Applications can assign the Authentication class to be used for the request. If Authentication has not been assigned, and BasicAuthentication is True, a TIdBasicAuthentication object instance will be automatically created in SetHeaders.