Proxy forwarding is being configured using an appliance other than a Proxy SG or Microsoft ISA/TMG. (i.e. Citrix Netscaler)
Web Security Service
Proxy forwarding is essentially an explicit proxy connection where HTTPS traffic is sent over 8080 and HTTP is sent over 8443. There are no authentication options for proxy forwarded traffic in the WSS portal. Authentication using the proxy forwarding access method is configured on the forwarding appliance. Symantec provides documentation covering authentication for ProxySG devices or Microsoft ISA/TMG devices. It is possible to use other devices with the proxy forwarding access method, however, Symantec doesn't offer support for other devices and so it is up to the customer to configure their appliance to provide the user/group information with the traffic.