Get custom property of signed in user in .NET Core

I'm working on a .NET Core project and I have added a custom column to the ASPNetUsers table called DisplayName that users set during the registration. I've successfully migrated the tables with that data and everything works fine.

However in my Login partial, I have to access the user's data through UserManager, and there's no method of getting that DisplayName INSTEAD of the Username.

What do I need to do to extend the function of the "normal" Identity manager to get the custom method to retrieve the text that's stored in DisplayName column?