explict proxy with fsso/ntlm authentication and fallback to Basic/Form

Hello! I have a customer where i need to use the FortiGate explicit proxy. Most part of the authentication is solved via FSSO/NTLM, but i need to use Basic/Form authentication as a fallback because some computers are not in the AD domain. In order to do this i took a look at authentication schemes and rules, but i dont understand how to implement it...

Regards,

HA