Change the default message in the Security warning prompt when adding the DefaultFileName property of SaveFileDialog in Silverlight5

I am using Silverlight5 and I just need to save a file from my Application. When user clicks the save button, it should open the SaveFileDialog with a default name. 
Using the following code:

    

private void Button_Click(object sender, RoutedEventArgs e)
        {
            SaveFileDialog dialog = new SaveFileDialog
            {
                DefaultExt = "xls",
                Filter =
                    String.Format("{1} files (*.{0})|*.{0}|All files (*.*)|*.*", "xls", "Excel"
                                  ),
                FilterIndex = 1,
                DefaultFileName = "Test",

            };
            dialog.ShowDialog();
        }

But the problem is, when we are adding DefaultFileName property to SaveFileDialog, it will show automatically a confirmation message window. I need to customize the message in the MessageBox shown when open SaveFileDialog.

Hope some of you guys may come across this problem, please guide me to overcome this one. 

Please find the image below