public class AppDbInitializer : DropCreateDatabaseAlways<ApplicationDbContext>
{
    protected override void Seed(ApplicationDbContext context)
    {
        var roleManager = new RoleManager<IdentityRole>(new RoleStore<IdentityRole>(context));

        // создаем две роли
        var role1 = new IdentityRole { Name = "admin" };
        var role2 = new IdentityRole { Name = "user" };

        // добавляем роли в бд
        roleManager.Create(role1);
        roleManager.Create(role2);

        context.SaveChanges();
        base.Seed(context);
    }
}

Database.SetInitializer<ApplicationDbContext>(new AppDbInitializer());