If you want to redirect traffic from the root of the domain to the Portal for ArcGIS homepage you'll need to set this up in the front-end web server.
Here's some relevant documentation for IIS: https://learn.microsoft.com/en-us/iis/configuration/system.webserver/httpredirect/
I used the following configuration and it seemed to be effective for my ArcGIS Enterprise deployment:
-- Chris Pawlyszyn