In my project, every client request will have a custom HTTP header that specifies the upstream proxy (gateway) the client wants to use. There is documentation on how to use an upstream proxy in FiddlerCore, but this seems to be a global setting, not per session.
If this is not possible in FiddlerCore, how can I achieve this by sending a new request to the proxy server (possibly cloning the original one and tampering with it), and do something similar with the response I will get back?
The solution must work with HTTPS too.