Chain Fiddler to an Upstream Proxy
To configure Fiddler to send and receive web traffic to and from another proxy between Fiddler and the destination server:
Open Internet Explorer > Options > Internet Options > Connections > LAN Settings.
Click the check box by Use a proxy server for your LAN.
Type the address and port number for the upstream proxy.
You should now see the upstream proxy listed in the Fiddler About dialog.