const string sessionKey = "FirstSeen";
            DateTime dateFirstSeen;
            var value = HttpContext.Session.GetString(sessionKey);
            if (string.IsNullOrEmpty(value))
            {
                dateFirstSeen = DateTime.Now;
                var serialisedDate = JsonConvert.SerializeObject(dateFirstSeen);
                HttpContext.Session.SetString(sessionKey, serialisedDate);
            }
            else
            {
                dateFirstSeen = JsonConvert.DeserializeObject<DateTime>(value);
            }
            var testSession = new TestSession()
            {
                DateSessionStarted = dateFirstSeen,
                Now = DateTime.Now
            };

app.UseCookiePolicy();

I did not create my own ApplicationUser class, because I decided to take the default IdentityUser, so I replaced it in views Login.cshtml and _LoginPartial.cshtml and also changed the AccountController (and ManageController) constructor as follows

public class Startup
    {
        public Startup(IHostingEnvironment env)
        {
            var builder = new ConfigurationBuilder()
                          .SetBasePath(env.ContentRootPath)
                          .AddJsonFile("appsettings.json", optional: false, reloadOnChange: true)
                          .AddJsonFile($"appsettings.{env.EnvironmentName}.json", optional: true)
                          .AddEnvironmentVariables();

            Configuration = builder.Build();
        }

        public IConfiguration Configuration { get; }

        // This method gets called by the runtime. Use this method to add services to the container.
        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.AddDbContext<DbContext>(options => options.UseMySql(Configuration.GetConnectionString("DefaultConnection")));
            services.AddScoped<BackEndContext>();

            services.AddMvc().SetCompatibilityVersion(CompatibilityVersion.Version_2_1);
        }

        // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
        public void Configure(IApplicationBuilder app, IHostingEnvironment env)
        {
            if (env.IsDevelopment())
            {
                app.UseDeveloperExceptionPage();
            }
            else
            {
                app.UseExceptionHandler("/Home/Error");
                app.UseHsts();
            }

            //app.UseHttpsRedirection();
            app.UseStaticFiles();
            app.UseCookiePolicy();

            app.UseMvc(routes =>
            {
                routes.MapRoute(
                    name: "area",
                    template: "{area:exists}/{controller=Admin}/{action=Index}/{id?}");

                routes.MapRoute(
                    name: "default",
                    template: "{controller=Home}/{action=Index}/{id?}");
            });
        }
    }

services.AddScoped<BackEndContext>();

services.AddDbContext<DbContext>(options => options.UseMySql(Configuration.GetConnectionString("DefaultConnection")));
services.AddScoped<BackEndContext>();

public class BackEndContext
    {
        public string ConnectionString { get; set; }

        public BackEndContext(IConfiguration configuration)
        {
            this.ConnectionString = configuration.GetConnectionString("DefaultConnection");
        }

        private MySqlConnection GetConnection() => new MySqlConnection(ConnectionString);

        public List<Utenti> GetAllUtenti()
        {
            Dictionary<string, object> p = new Dictionary<string, object>();

            List<Utenti> list = SQLHelper.FillByQueryName(enQueries.BackEnd_Utenti, p, GetConnection()).ConvertDataTableToGenericList<Utenti>();

            return list;
        }
    }