private static HttpClientHandler CreateClientHandler(ProxyConfig proxyConfig = null)
        {
            HttpClientHandler clientHandler = new HttpClientHandler
            {
                PreAuthenticate       = false,
                UseDefaultCredentials = true,
                UseProxy = false
            };

            if (!string.IsNullOrWhiteSpace(proxyConfig?.Host))
            {
                WebProxy proxy = proxyConfig.CreateWebProxy();
                clientHandler.UseProxy = true;
                clientHandler.Proxy    = proxy;
                clientHandler.UseDefaultCredentials = proxy.UseDefaultCredentials;
                clientHandler.PreAuthenticate       = proxy.UseDefaultCredentials;
            }

            return(clientHandler);
        }