Startup.cs

public void ConfigureServices(IServiceCollection services)
{
    services.Configure<CookiePolicyOptions>(options =>
    {
        // This lambda determines whether user consent for non-essential cookies is needed for a given request.
        options.CheckConsentNeeded = context => true;
        options.MinimumSameSitePolicy = SameSiteMode.None;
    });
    services.AddAuthentication(CookieAuthenticationDefaults.AuthenticationScheme).AddCookie(cookieOptions => {
        cookieOptions.LoginPath = "/";
    });

    services.AddMvc().AddRazorPagesOptions(options => {
        options.Conventions.AuthorizeFolder("/admin");
    }).SetCompatibilityVersion(CompatibilityVersion.Version_2_2);
}