Startup.cs

using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.Extensions.DependencyInjection;
using DNTCaptcha.Core;
using Microsoft.AspNetCore.Mvc;
using Microsoft.Extensions.Hosting;
using Microsoft.AspNetCore.Routing;

namespace DNTCaptcha.TestWebApp
{
    public class Startup
    {
        public void ConfigureServices(IServiceCollection services)
        {
            services.AddCors(options =>
            {
                options.AddPolicy("CorsPolicy",
                    builder => builder
                        .WithOrigins("http://localhost:4200") //Note:  The URL must be specified without a trailing slash (/).
                        .AllowAnyMethod()
                        .AllowAnyHeader()
                        .SetIsOriginAllowed((host) => true)
                        .AllowCredentials());
            });
            services.AddMvc(options => options.Filters.Add(new AutoValidateAntiforgeryTokenAttribute()));

            services.Configure<RouteOptions>(o => o.LowercaseUrls = true);


            // Added it for AddDNTCaptcha -> options.UseDistributedCacheStorageProvider()
            /*services.AddStackExchangeRedisCache(options =>
            {
                options.Configuration = "localhost:6379";
                options.InstanceName = "test";
            });*/

            services.AddDNTCaptcha(options =>
            {
                // options.UseSessionStorageProvider(); // -> It doesn't rely on the server or client's times. Also it's the safest one.
                // options.UseMemoryCacheStorageProvider(); // -> It relies on the server's times. It's safer than the CookieStorageProvider.
                options.UseCookieStorageProvider(); // -> It relies on the server and client's times. It's ideal for scalability, because it doesn't save anything in the server's memory.
                // options.UseDistributedCacheStorageProvider(); // --> It's ideal for scalability using `services.AddStackExchangeRedisCache()` for instance.
                // options.UseDistributedSerializationProvider();
            });

            services.AddSession();

            services.AddControllersWithViews();
            services.AddRazorPages();
        }

        public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
        {
            if (env.IsDevelopment())
            {
                app.UseDeveloperExceptionPage();
            }
            else
            {
                app.UseExceptionHandler("/Home/Error");
                app.UseHsts();
            }
            app.UseHttpsRedirection();

            app.UseCors(policyName: "CorsPolicy");

            app.UseStaticFiles();

            app.UseRouting();

            app.UseSession();

            app.UseEndpoints(endpoints =>
            {
                endpoints.MapControllerRoute(
                    name: "default",
                    pattern: "{controller=Home}/{action=Index}/{id?}");
                endpoints.MapRazorPages();
            });
        }
    }
}