I found this link:
How to get the current logged in user Id ASP.NET Core
But the answers are all OUTDATED!
And this SO post can not neither be a solution, its just a boilerplate workaround: How should I access my ApplicationUser properties from within my MVC 6 Views?
In the mvc action: base.User is not of type ApplicationUser neither is base.User.Identity. Both can not be casted into ApplicationUser.
So how can I access my custom properties put inside the applicationuser object when logged in?