Keep the Proxy-Configured GoogleCredential Copy in .NET

var proxy = new System.Net.WebProxy("http://proxy.example:8080")
{
    Credentials = proxyCredentials
};
var factory = Google.Apis.Http.HttpClientFactory.ForProxy(proxy);
var routedCredential = credential.CreateWithHttpClientFactory(factory);

credential is the application's existing GoogleCredential. The method returns a copy, so use routedCredential for token work or API authentication instead of continuing with the original.

proxyCredentials identifies the intermediary account; omit the initializer if none is needed. In that unauthenticated case, the HTTP proxy list can supply endpoint candidates when the token destination is public and a public-proxy route is permitted. Preserve the Google credential's required type and scopes. Proxy credentials do not substitute for a client secret, service-account key or access token.

The API service client is another construction path. Give that transport the intended factory too when both service and token requests need the proxy. Configuring the service alone can still leave an earlier token request unable to connect.

A cached access token avoids a new exchange. Compare the actual failed host and request rather than treating a cached-token operation or object construction as a new proxy connection.