using System;

public class Program

{

    public static void Main()

    {

        var a = BitConverter.GetBytes((ushort)3);

        var b = Convert.FromBase64String(@"VC//UiExBNwDhjkr1znBSDG0cDeMY90g");

​

        byte[] buffer = new byte[a.Length + b.Length];

        Array.Copy(a, buffer, a.Length);

        Array.Copy(b, 0, buffer, 2, b.Length);

        //buffer.Dump();

        Console.WriteLine(Convert.ToBase64String((buffer)));

    }

}