Sorry, I didnt explain properly my question. We have 2 wan interfaces to Internet and separately aceess (different access to internet through education local network but only through proxy).
I could say we have 3 ways to access internet : 2 direct wans and one through proxy.
We configured one FG interface to connect to squid box (squid box has proxy with education parent proxy), and this interface works as part of SD-WAN fine. We would like to use only FG (dont use external squid box at all).