How to redirect traffic in Mikrotik to the correct port?

Question

VPN

Hello. There is a task like as trivial and in too time not absolutely.
Given:
We have a head office with a domain and a proxy server. All domain users access the Internet through the Traffic Inspector proxy, which is installed on the domain server.
The domain server has two network cards. One looks to the local area, the other to the mikrotik, which is connected to the Internet.
Recently, a remote office has appeared, which I also want to bring into the domain (already started) and distribute the Internet to them through a proxy (but this is a problem)
In order to make everything clear, I give a diagram.
At the moment, the situation is as follows:
A VPN server has been raised on Mikrotik, to which Zyxel Giga2 can successfully connect via PPTP through the provider's local network.
I see remote machines, they see the domain, they are even brought into it, but the Internet is taken directly from Mikrotik. 6,7,8 ports on Mikrotik are isolated from others.
How to make it so that the VPN, raised on the 4th port of the Mikrotik, does not see the Internet of the Mikrotik itself and climbs behind it on the proxy server, that is, through the 5th port of the Mikrotik.
I hope I described the question more or less clearly. I need an answer, at least in theory, in which direction to dig and with what help to implement. Or maybe something to alter in this scheme.

Answer 1

Try to mark traffic (mangle) from a remote network on Mikrotik using the prerouting chain and then create a default route (0.0.0.0) for traffic with this label, and specify the proxy address as the gateway

Answer 2

Isn't it easier in this case to simply distribute all proxies forcibly in politics?!)
And just in case, kill the forward (except for locales) on the 4th interface in 0.0.0.0/0?

Answer 3

polyrouting:
route brand in routes, mrka in mangles, use of address sheets, and firewall rules based on mangles - done.
No, well, if it is difficult to describe - ask what exactly is difficult.