using Titanium.Web.Proxy;
...
var proxyServer = new ProxyServer();

// locally trust root certificate used by this proxy 
proxyServer.CertificateManager.TrustRootCertificate(true);

// optionally set the Certificate Engine
// Under Mono only BouncyCastle will be supported
//proxyServer.CertificateManager.CertificateEngine = Network.CertificateEngine.BouncyCastle;

proxyServer.BeforeRequest += OnRequest;
proxyServer.BeforeResponse += OnResponse;
proxyServer.ServerCertificateValidationCallback += OnCertificateValidation;
proxyServer.ClientCertificateSelectionCallback += OnCertificateSelection;


var explicitEndPoint = new ExplicitProxyEndPoint(IPAddress.Any, 8000, true)
{
    // Use self-issued generic certificate on all https requests
    // Optimizes performance by not creating a certificate for each https-enabled domain
    // Useful when certificate trust is not required by proxy clients
   //GenericCertificate = new X509Certificate2(Path.Combine(System.IO.Path.GetDirectoryName(System.Reflection.Assembly.GetExecutingAssembly().Location), "genericcert.pfx"), "password")
};

// Fired when a CONNECT request is received
explicitEndPoint.BeforeTunnelConnectRequest += OnBeforeTunnelConnectRequest;

// An explicit endpoint is where the client knows about the existence of a proxy
// So client sends request in a proxy friendly manner
proxyServer.AddEndPoint(explicitEndPoint);
proxyServer.Start();

// Transparent endpoint is useful for reverse proxy (client is not aware of the existence of proxy)
// A transparent endpoint usually requires a network router port forwarding HTTP(S) packets or DNS
// to send data to this endPoint
var transparentEndPoint = new TransparentProxyEndPoint(IPAddress.Any, 8001, true)
{
    // Generic Certificate hostname to use
    // when SNI is disabled by client
    GenericCertificateName = "google.com"
};

proxyServer.AddEndPoint(transparentEndPoint);

//proxyServer.UpStreamHttpProxy = new ExternalProxy() { HostName = "localhost", Port = 8888 };
//proxyServer.UpStreamHttpsProxy = new ExternalProxy() { HostName = "localhost", Port = 8888 };

foreach (var endPoint in proxyServer.ProxyEndPoints)
Console.WriteLine("Listening on '{0}' endpoint at Ip {1} and port: {2} ",
    endPoint.GetType().Name, endPoint.IpAddress, endPoint.Port);

// Only explicit proxies can be set as system proxy!
proxyServer.SetAsSystemHttpProxy(explicitEndPoint);
proxyServer.SetAsSystemHttpsProxy(explicitEndPoint);

// wait here (You can use something else as a wait function, I am using this as a demo)
Console.Read();

// Unsubscribe & Quit
explicitEndPoint.BeforeTunnelConnectRequest -= OnBeforeTunnelConnectRequest;
proxyServer.BeforeRequest -= OnRequest;
proxyServer.BeforeResponse -= OnResponse;
proxyServer.ServerCertificateValidationCallback -= OnCertificateValidation;
proxyServer.ClientCertificateSelectionCallback -= OnCertificateSelection;

proxyServer.Stop();

private async Task OnBeforeTunnelConnectRequest(object sender, TunnelConnectSessionEventArgs e)
{
    string hostname = e.HttpClient.Request.RequestUri.Host;

    if (hostname.Contains("dropbox.com"))
    {
         // Exclude Https addresses you don't want to proxy
         // Useful for clients that use certificate pinning
         // for example dropbox.com
         e.DecryptSsl = false;
    }
}

public async Task OnRequest(object sender, SessionEventArgs e)
{
    Console.WriteLine(e.HttpClient.Request.Url);

    // read request headers
    var requestHeaders = e.HttpClient.Request.Headers;

    var method = e.HttpClient.Request.Method.ToUpper();
    if ((method == "POST" || method == "PUT" || method == "PATCH"))
    {
        // Get/Set request body bytes
        byte[] bodyBytes = await e.GetRequestBody();
        e.SetRequestBody(bodyBytes);

        // Get/Set request body as string
        string bodyString = await e.GetRequestBodyAsString();
        e.SetRequestBodyString(bodyString);
    
        // store request 
        // so that you can find it from response handler 
        e.UserData = e.HttpClient.Request;
    }

    // To cancel a request with a custom HTML content
    // Filter URL
    if (e.HttpClient.Request.RequestUri.AbsoluteUri.Contains("google.com"))
    {
        e.Ok("<!DOCTYPE html>" +
            "<html><body><h1>" +
            "Website Blocked" +
            "</h1>" +
            "<p>Blocked by titanium web proxy.</p>" +
            "</body>" +
            "</html>");
    }

    // Redirect example
    if (e.HttpClient.Request.RequestUri.AbsoluteUri.Contains("wikipedia.org"))
    {
        e.Redirect("https://www.paypal.com");
    }
}

// Modify response
public async Task OnResponse(object sender, SessionEventArgs e)
{
    // read response headers
    var responseHeaders = e.HttpClient.Response.Headers;

    //if (!e.ProxySession.Request.Host.Equals("medeczane.sgk.gov.tr")) return;
    if (e.HttpClient.Request.Method == "GET" || e.HttpClient.Request.Method == "POST")
    {
        if (e.HttpClient.Response.StatusCode == 200)
        {
            if (e.HttpClient.Response.ContentType != null && e.HttpClient.Response.ContentType.Trim().ToLower().Contains("text/html"))
            {
                byte[] bodyBytes = await e.GetResponseBody();
                e.SetResponseBody(bodyBytes);

                string body = await e.GetResponseBodyAsString();
                e.SetResponseBodyString(body);
            }
        }
    }
    
    if (e.UserData != null)
    {
        // access request from UserData property where we stored it in RequestHandler
        var request = (Request)e.UserData;
    }
}

// Allows overriding default certificate validation logic
public Task OnCertificateValidation(object sender, CertificateValidationEventArgs e)
{
    // set IsValid to true/false based on Certificate Errors
    if (e.SslPolicyErrors == System.Net.Security.SslPolicyErrors.None)
        e.IsValid = true;

    return Task.CompletedTask;
}

// Allows overriding default client certificate selection logic during mutual authentication
public Task OnCertificateSelection(object sender, CertificateSelectionEventArgs e)
{
    // set e.clientCertificate to override
    return Task.CompletedTask;
}