Web proxy in AppVM

[JPL]

I want to set up a web proxy in a Debian-based AppVM but I don't want to do it in the template VM as last time I tried that I screwed lots of things up and it was never the same again.

I don't mind if it's not persistent because I am just experimenting. Lacking the usual Gnome systems tool I don't how how to do it. I have a URL of a PAC file http://xyz.pac on the proxy server I want to use. 

Can someone tell me how to do this please?

Thanks