RemotePost doesn't work properly. (4.2)

 public void PostProcessPayment(PostProcessPaymentRequest postProcessPaymentRequest)
        {
            
            RemotePost remotePost = new RemotePost();
            remotePost.Url = "*************";
            remotePost.Add("clientid", "*************");
            remotePost.Add("clientIp", "*************");
            remotePost.Add("storetype", "*************");
            remotePost.Add("hash", "*************");
            remotePost.Add("islemtipi", "*************");
            remotePost.Add("amount", "*************");
            remotePost.Add("currency", "*************");
            remotePost.Add("oid", "*************");
            remotePost.Add("okUrl", "*************");
            remotePost.Add("taksit", "*************");
            remotePost.Add("rnd", "*************");
            remotePost.Add("failUrl", "*************");

            remotePost.Post();
        }

                    if (_webHelper.IsRequestBeingRedirected || _webHelper.IsPostBeingDone)
                    {
                        var _httpContextAccessor = EngineContext.Current.Resolve<IHttpContextAccessor>();
                       
                        if (_httpContextAccessor.HttpContext.Session.TryGetValue("RedirectToThirdParty",out _))
                        {
                            var value = _httpContextAccessor.HttpContext.Session.Get<string>("RedirectToThirdParty");
                            if (!string.IsNullOrEmpty(value))
                            {
                                return Content(value, @"text/html");
                            }
                           
                        }
                        //redirection or POST has been done in PostProcessPayment
                        return Content("Redirected");
                    }

            //post
            var httpContext = _httpContextAccessor.HttpContext;
            var response = httpContext.Response;
            response.Clear();
            var data = Encoding.UTF8.GetBytes(sb.ToString());

            response.OnStarting(state =>
            {
                var context = (HttpContext)state;
                context.Response.Headers.Add("Content-Type", "text/html; charset=utf-8");
                context.Response.Headers.Add("Content-Length", data.Length.ToString());

                return Task.CompletedTask;
            }, httpContext);

            response.Body.Write(data, 0, data.Length);

            //store a value indicating whether POST has been done
            _webHelper.IsPostBeingDone = true;