Custom RoleProvider in ASP.NET Core with Identity?

In past MVC versions I was able to do

<roleManager enabled="true" defaultProvider="...." ...

in to the web.config to get a custom role provider, but that doesn't seem to be the case anymore.

Essentially what I want to do is:

1. The user logs in.
2. On success, get roles for user from external source.
3. Apply roles to user to be used in code.
4. Match user roles to roles in custom RoleProvider

How do I do this in ASP.NET Core?