HttpWebRequest via C#

so is you can see down here I'm using random to change the account info but I've problem with the email I don't want it to change the email and if I delete it it won't work :S so how I can make it not change the email and leave alone ?

byte[] postData = Encoding.ASCII.GetBytes(string.Format("csrfmiddlewaretoken={0}&first_name={1}&email={2}%40gmail.com&username={3}&phone_number=&gender=3&biography=&external_url=&chaining_enabled=on",

class ThreadSafeRandom
{

    private static Random random = new Random();
    private static string _chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";

    private static string _nums = "0123456789";
    public static string NextString(int size = 8)
    {
        lock (random)
        {
           return new string(Enumerable.Repeat(_chars, size)
                   .Select(s => s[random.Next(s.Length)]).ToArray());



        }
    }

    public static int Next(int low, int max)
    {
        lock (random)
        {
            return random.Next(low, max);
        }
    }
}
class Instagram
{

    private CookieContainer s0;
    public string proxy = "";
    private const string csrfPattern = "csrf_token\":\"(.*?)\"";

    public Instagram()
    {
        s0 = new CookieContainer();
    }
    public bool c = false;
    public void Claim(object username)
    {
        byte[] postData = Encoding.ASCII.GetBytes(string.Format("csrfmiddlewaretoken={0}&first_name={1}&email={2}%40gmail.com&username={3}&phone_number=&gender=3&biography=&external_url=&chaining_enabled=on",
            GetCSRF2(), ThreadSafeRandom.NextString(), ThreadSafeRandom.NextString(), username.ToString()));