Define 2 roles for our ASP.NET Core MVC 6.0 (Admin & Users). if we do not have registration and we are authenticating users using MS Identity Manager

Question

I created a new ASP.NET Core MVC 6.0 web application >> and i define it to use Microsoft Identity Platform for authentication, as follow:-

so we do not manage or register users inside back-end. now we need to define all users to have "Users" role while certain users to have "Admin" role. so how we can do so? since we are not managing our users inside SQL ? can anyone advice what are the approaches we have? now we can manage defining users with Admin roles on the back-end only (without having views to manage this).. but not sure which approach we need to follow?

Thanks