How to MD5 encrypt using Challenge Key

Question

I am trying to establish a connection to a Server using MD5 Authentication. The server returns me the challenge key. How can I use it to encrypt my password and send it back to the server. The encryption method currently I know needs Password Hash, Salt Key, and VI Code. How Can I make it .

    static readonly string PasswordHash = "PasswordHash";
    static readonly string SaltKey = "SaltKey";
    static readonly string VIKey = "@1B2c3D4e5F6g7H8";

    public static string Encrypt(string plainText)
    {
        byte[] plainTextBytes = Encoding.UTF8.GetBytes(plainText);

        byte[] keyBytes = new Rfc2898DeriveBytes(PasswordHash, Encoding.ASCII.GetBytes(SaltKey)).GetBytes(256 / 8);
        var symmetricKey = new RijndaelManaged() { Mode = CipherMode.CBC, Padding = PaddingMode.Zeros };
        var encryptor = symmetricKey.CreateEncryptor(keyBytes, Encoding.ASCII.GetBytes(VIKey));

        byte[] cipherTextBytes;

        using (var memoryStream = new MemoryStream())
        {
            using (var cryptoStream = new CryptoStream(memoryStream, encryptor, CryptoStreamMode.Write))
            {
                cryptoStream.Write(plainTextBytes, 0, plainTextBytes.Length);
                cryptoStream.FlushFinalBlock();
                cipherTextBytes = memoryStream.ToArray();
                cryptoStream.Close();
            }
            memoryStream.Close();
        }
        return Convert.ToBase64String(cipherTextBytes);