Proxy authentication problem WPF browser app calling WCF service

Question

I have a WPF browser app (partial trust) that calls a WCF service that runs
on the web server. All works fine when the client is not behind a proxy
server. If it is, I get the message: "(407) Proxy Authentication Required".
If I try to alter the <defaultProxy> settings I get the message "Insufficient
permissions for setting the configuration section 'defaultProxy'". What
should I do? I just want to use the same credentials for normal web access
and WCF access.