The file you'll need to edit is /etc/proxycache.conf

At the bottom of this file, you'll find the line:

http_port 3128

You'l want to replace that line with the following line:

http_port 192.168.0.1:3128 10.1.2.3:3128

Of course, you'll want to use your IP addresses, and not the ones listed.

Then, to stop NetMAX from overwriting these changes, you'll need to edit the /etc/commit.conf file, and add the following line:

skip /etc/proxycache.conf

If you ever need to make any changes to your proxy server configuration in the NetMAX Interface, you'll want to remove the line from the /etc/commit.conf file, commit your changes, then perform the above steps again.
address-suppressed