First of all you need to start visual studio as admin.
Put this code on add button click :
protected void btnAdd_Click(object sender, EventArgs e)
{
Configuration config = WebConfigurationManager.OpenWebConfiguration(HttpContext.Current.Request.ApplicationPath);
//Configuration config = WebConfigurationManager.OpenWebConfiguration("~");
config.AppSettings.Settings.Add(txtKey.Text, txtValue.Text);
config.Save();
msgLabel.Text = "key added in config file";
txtKey.Text = null;
txtValue.Text = null;
}
Put this code on delete button after entering key name in textbox.
protected void btnDelete_Click(object sender, EventArgs e)
{
Configuration config = WebConfigurationManager.OpenWebConfiguration(HttpContext.Current.Request.ApplicationPath);
config.AppSettings.Settings.Remove(txtKey.Text);
config.Save();
msgLabel.Text = "key deleted from config file";
txtKey.Text = null;
}