replacing the link in html page in mail body

Question

i have designed a mailer which contains a html page as mail body. There are certain links in the html page. There can be any number of links and i don't know about them. since everything inside a html page is considered a string. What i want is whenever i click on a particular link, that link should be replaced with a link to an intermediate page where i want the parameter values through query string. Here is my code-

protected void btnSubmit_Click(object sender, EventArgs e)
        {
            {
                SendHTMLMail();
            }


            void SendHTMLMail()
            {
                StreamReader reader = new StreamReader(Server.MapPath("~/index1.html"));
                string readFile = reader.ReadToEnd();
                string myString = "http";
                myString = readFile;
                myString = myString.Replace(@"??", @"http://localhost:60488/url.aspx?" + "sender=" + Server.UrlEncode(this.txtUsername.Text) + "&reciever=" + Server.UrlEncode(this.txtTo.Text));


                MailMessage Msg = new MailMessage();

                Msg.From = new MailAddress(txtUsername.Text);

                Msg.To.Add(txtTo.Text);
                Msg.Subject = txtSubject.Text;
                Msg.Body = myString.ToString();
                Msg.IsBodyHtml = true;

                if (fuAttachment.HasFile)
                {
                    string FileName = Path.GetFileName(fuAttachment.PostedFile.FileName);

                    Msg.Attachments.Add(new Attachment(fuAttachment.PostedFile.InputStream, FileName));
                }

                SmtpClient smtp = new SmtpClient();
                smtp.Host = "smtp.gmail.com";
                smtp.Port = 587;
                smtp.UseDefaultCredentials = false;
                smtp.Credentials = new System.Net.NetworkCredential(txtUsername.Text, txtpwd.Text);
                smtp.EnableSsl = true;
                smtp.Send(Msg);
                Msg = null;
                ClientScript.RegisterStartupScript(GetType(), "alert", "alert('Email sent.');", true);
            }

        }

in this block-

StreamReader reader = new StreamReader(Server.MapPath("~/index1.html"));
                    string readFile = reader.ReadToEnd();
                    string myString = "http";
                    myString = readFile;
                    myString = myString.Replace(@"??", @"http://localhost:60488/url.aspx?" + "sender=" + Server.UrlEncode(this.txtUsername.Text) + "&reciever=" + Server.UrlEncode(this.txtTo.Text));

what i tried is considered the url as a string and given it an initial value "http". Since i don't know anything after http as it can be any link for example - http://example.co.in or https://example.com i want to replace the link with the link of my intermediate page and also want to know the complete link after http and send that link name in a query string