It can not be done once you federate a server with portal you are affected by a bug. BUG-000092043 : The GeoEvent Processor in a federated ArcGIS for Server and Portal for ArcGIS environment does not allow users to login to environments which utilizes a DNS name for their Portal site. When accessing the GeoEvent Processor login page, it prompts for Portal credentials, and even when using a PSA account to log in, the site displays a "The username or password you've entered is incorrect" error. The network traffic logged the following error "proxy?https://dns.domain.com/webadaptorsharing/generateToken", which is in relation to the site being unable to generate a token as it is missing a '/' between the ArcGIS Web Adaptor and the sharing identifiers in the URL.
... View more