Unlike FiddlerCore, FiddlerCore for .NET Standard does not change the system proxy settings to point to the localhost port which it opens by default. The reason for this change is that FiddlerCore for .NET Standard is a library which could be used on many platforms in various ways and it could not predict how exactly the user would like to set up its proxy settings.
It is expected that the user will change the proxy settings for all applications whose traffic must pass trough FiddlerCore for .NET Standard after it is started and revert the proxy settings of these applications before FiddlerCore for .NET Standard is stopped. (either manually or programmatically)
However, if you are running on Windows platform, you could add the RegisterAsSystemProxy flag to the FiddlerCoreStartupFlags enumeration which you are passing to the FiddlerApplication.Startup method. Please, note that for all other platforms this will throw a PlatformNotSupported exception.
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