detect real source ip behind external proxy server

Hi guys,

i have a network with external web proxy which is before the fortigate,

now all the traffic that came from the proxy is with the proxy source ip, i want to detect the real source ip of the workstation that make a connection.

the web proxy is configured to send xff (X-Forwarded-For ) but what do i need to configure in FGT to received that?

Thank you!