System.Configuration.Configuration conf = System.Web.Configuration.WebConfigurationManager.OpenWebConfiguration("~");
int Cnt = System.Configuration.ConfigurationManager.AppSettings.Count;
string Key = "Name" + Cnt.ToString();
string Value = DateTime.Now.ToLongDateString() +
" " + DateTime.Now.ToLongTimeString();

conf.AppSettings.Settings.Add(Key, Value);

conf.Save(System.Configuration.ConfigurationSaveMode.Modified);