Pick FiddlerCore's Parent Proxy in BeforeRequest

FiddlerCore can forward a session through a parent proxy selected by the application. Set the session's x-overrideGateway flag in BeforeRequest:

FiddlerApplication.BeforeRequest += session => {
    session["x-overrideGateway"] = "proxy.office.example:8080";
};

This is a FiddlerCore event-registration fragment for an existing host application. The flag's ordinary host:port form identifies an HTTP gateway. The request's destination remains the website it originally named. For an independent public-site test with no parent-proxy credentials configured, the HTTP proxy list offers candidate gateway host:port values when the network permits that route.

An unconditional handler applies the same parent to sessions passing through it. For selective routing, put the assignment inside the application's destination rule, leaving other sessions to their intended gateway policy.

The documented special value DIRECT selects a direct route for that session. That is useful when an internal destination should be reached without the corporate parent.

There are two listeners to keep straight: clients connect to the FiddlerCore listener, while the override identifies the next proxy. Pointing the override back at FiddlerCore's own listening endpoint would send the request around a loop.

Set the flag before the request is sent upstream. Changing it after the response has arrived can change displayed session data, but cannot reroute the completed network operation.