(Windows) Active Directory Username shortcode

We are implementing Formidable Pro in some projects at my job and we decided we didn't want to require users to have a wordpress account to use our system. The following shortcode pulls the users login credentials (username):

//create [aduser] shortcode
function aduser_shortcode( $atts ){
$user = getenv('username');
return $user;
}
add_shortcode( 'aduser', 'aduser_shortcode' );

Now if you put the shortcode "[aduser]" into a field as the default value, it will populate the currently logged in username in the active directory.