Proxy settings for VPN clients

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Hello,

I have FortiGate 200D configured as SSL VPN Gate for remote users. Fortigate acts like "router on the stick" - its hidden behind firewall, Nat'd for public address (to be accessible from Internet) and connected to the rest of infrastructure only using one interface.

Remote users of PC, don't have any problems, because in OS (or browser) settings they can set the proxy server address, that is used inside of the company.

The problem is that mobile users (IOS, Android) cannot set the proxy address in Forticlient APP.

I don't want to setup additional transparent proxy.

How can I solve this?

I thought about setting WCCP for only VPN clients, and redirect HTTP and HTTPS to proxy, but I cannot find how to do this. What more - I'm not sure if it's possible to distinguish remote mobile clients from remote PC clients.

Second thought was to setup additional IPSEC VPN for mobile clients, because in native Android and IOS vpn clients you can set the proxy addresses.

Maybe someone faced same problem? How to solve that on Fortigate?