Using Bob Alfsons' guide as a reference, https://support.astaro.com/support/index.php/User_Contributed:HTTP/S_Proxy_Access_with_AD-SSO .
See section V of the linked document.
I was wondering if this has been fixed for V8? Unfortunately, my beta V8 install is not in an AD domain environment to check. Getting this changed, so that the DN is used would bring it in line and consistent with other areas.