default prompt for Integrated Windows Authentication is fqdn of proxy\username and this causes confusion for guest users using non-domain computers because this default prompt will not work even if the correct password is entered.
To succesfully login they must modify the prompt.
by default the prompt is sent as: proxy1.example.com\username
user must then know to change to example\username
customer would like to have ability to change the prompt so all the user has to do is enter the password