There's a few different ways to tag user sessions with more meaningful data like usernames. I would refer you to this page on the doc which goes into detail on how to tag sessions -> https://www.dynatrace.com/support/help/user-experi...
Hope that helps.
Go to the User Tags section for your application and you should be able to see 'cookie value' in the dropdown. I've attached a screenshot from the doc I linked previously showing this as well.