How to configure .NET 4.0 (WPF) app to use system default proxy server (authentication sometime required)?

Question

I had a .NET app which talk to a FTP server regularly to check for new files.

Sometime, the application runs behind a proxy server. I have implemented with reference to MSDN:

http://msdn.microsoft.com/en-us/library/dkwyc043(v=vs.100).aspx

<system.net>
    <defaultProxy>
              useDefaultCredentials="true"
              <proxy usesystemdefault="true" />
      />
    </defaultProxy>
    <settings>
        <webProxyScript downloadTime="00:00:30" />
    </settings>
<system.net>

But in one case, the proxy servers require authentication by user name and password. When I try opening the app, it cannot connect to Internet and the log said "Unable to connect to Internet, server required authentication".

How do I configure my .NET 4.0 app to work for all types of proxy servers (sometime authentication required) ?

PS: I read this thread: How should I set the default proxy to use default credentials?, however it mentioned default proxy & default credentials. In my case, the authentication can be in the form of username/password