The link doesn’t tell me how to set the VM.
The proxy software must run either:
under the linux user account tunnel on Whonix-Gateway ™; or
on the host; or
in another virtual machine.
I try to have both NAT and internal adapter on my VM, and set the internal to Whonix network, and forward the traffic from Whonix-gateway to VM’s internal adapter to proxy software by iptables, the proxy is running, but Whonix-gateway is not connection.