Forward web requests to a transparent web proxy with client IP address

[kirb]

We have encountered the following issue. We need to redirect all HTTP/HTTPS requests that come to the Fortigate to a remote proxy server, while maintaining the ability to authenticate users. We followed this guide to configure the Fortigate: https://docs.fortinet.com/document/fortigate/7.0.0/new-features/23601/selectively-forward-web-reques....
We managed to redirect all HTTP/HTTPS requests to the remote proxy server, but the problem is that Fortigate sends all requests to the remote proxy server with its own IP address. That is, all packets that were redirected had the Fortigate's IP address in the Source address field (we observed this in the proxy server logs). As a result, the proxy server cannot authenticate the user by IP address or by username. Does Fortigate have any solution to this problem?
Thank you in advance

[AEK]

Try with policy routes.

FG menu: Network > Policy Routes.