In Portal add a default Level 3 - Server Administrator user. The user name associated with this account would be set during installation. This should be thought of as a 'built' in user. One of these would come with each ArcGIS Enterprise Server license and will allow a user to install, administer, and test the software prior to going through the provisioning step required for level 1 and level 2 users.
Among other things this would facilitate EDN/ArcGIS Developer Subscription use of Portal and server.