To configure Fiddler Classic to send and receive web traffic to and from another proxy between Fiddler Classic and the destination server:
Close Fiddler Classic application.
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.
Restart Fiddler Classic application.
You should now see the upstream proxy listed in the Fiddler Classic About dialog.