public class CustomFlurlHttpClient : DefaultHttpClientFactory {
    public override HttpClient CreateClient(Url url, HttpMessageHandler m) {
        return base.CreateClient(url, CreateProxyHttpClientHandler("http://192.168.0.103:9090"));
    }

    private HttpClientHandler CreateProxyHttpClientHandler(string proxyUrl, string user = "", string passw = "") {
        NetworkCredential proxyCreds = null;
        var proxyUri = new Uri(proxyUrl);
        proxyCreds = new NetworkCredential (user, passw);
        var proxy = new WebProxy (proxyUri, false) {
            UseDefaultCredentials = false,
            Credentials = proxyCreds
        };
        var clientHandler = new HttpClientHandler {
            UseProxy = true,
            Proxy = proxy,
            PreAuthenticate = true,
            UseDefaultCredentials = false
        };
        if (user != "" && passw != "") {
            clientHandler.Credentials = new NetworkCredential (user, passw);
        }
        return clientHandler;
    }
}
class MainClass {
    public static void Main (string[] args) {
        run ();
        Console.ReadKey ();
    }

    async static void run() {
        using(FlurlClient client = new FlurlClient(c => { c.HttpClientFactory = new CustomFlurlHttpClient();})) {
            var result = await client.WithUrl("https://www.google.com").GetStringAsync();
            Console.WriteLine(result);
        };
    }
}

public override HttpClient CreateClient(Url url, HttpMessageHandler m)
    {
        var socksProxy = new Socks5ProxyClient("127.0.0.1", 9150);
        var handler = new ProxyHandler(socksProxy);
        return base.CreateClient(url, handler);
    }

public class CustomFlurlHttpClient : DefaultHttpClientFactory
{
    public override HttpMessageHandler CreateMessageHandler() {
        var socksProxy = new Socks5ProxyClient("127.0.0.1", 9150);
        return new ProxyHandler(socksProxy);
    }
}