// https://docs.microsoft.com/en-us/aspnet/core/security/authentication/scaffold-identity?view=aspnetcore-2.2&tabs=visual-studio
public void ConfigureServices(IServiceCollection services)
{
    services.Configure<CookiePolicyOptions>(options =>
    {
        options.CheckConsentNeeded = context => true;
        options.MinimumSameSitePolicy = SameSiteMode.None;
    });

    services.AddDbContext<ApplicationDbContext>(options =>
        options.UseSqlServer(
            Configuration.GetConnectionString("DefaultConnection")));

    services.AddIdentity<IdentityUser, IdentityRole>()
        // services.AddDefaultIdentity<IdentityUser>()
        .AddEntityFrameworkStores<ApplicationDbContext>()
        .AddDefaultTokenProviders();

    services.AddMvc().SetCompatibilityVersion(CompatibilityVersion.Version_2_1)
        .AddRazorPagesOptions(options =>
        {
            options.AllowAreas = true;
            options.Conventions.AuthorizeAreaFolder("Identity", "/Account/Manage");
            options.Conventions.AuthorizeAreaPage("Identity", "/Account/Logout");
        });

    services.ConfigureApplicationCookie(options =>
    {
        options.LoginPath = $"/Identity/Account/Login";
        options.LogoutPath = $"/Identity/Account/Logout";
        options.AccessDeniedPath = $"/Identity/Account/AccessDenied";
    });

    // using Microsoft.AspNetCore.Identity.UI.Services;
    services.AddSingleton<IEmailSender, EmailSender>();
}

// create EmailSender service
public class EmailSender : IEmailSender
{
    public Task SendEmailAsync(string email, string subject, string message)
    {
        return Task.CompletedTask;
    }
}

// and scaffold identity razor templates