I don't have a proxy environment to test so not sure if this will solve it but, a little googling produced this suggestion for settings in Web.config to setup a default proxy.
usessystemdefault = "false"
proxyaddress = "http://126.96.36.199:3128"
bypassonlocal = "true"
System.Net section is near the bottom of the mojoportal Web.config file. Obviously you would put in the address for your own proxy server.
Please let me know if it works or not.