You should probably specify the language you're using, but I'm going to assume that's C# and that you're using Visual Studio. I also don't fully understand what you're trying to do, but I'm pretty sure you want to get a byte array that contains the ASCII values of characters in a text box.
using Microsoft.VisualBasic;
char[] Characters = Textbox.Text.ToCharArray();
byte[] Bytes = new byte[Characters.Length - 1];
for (int i = 0; i < Characters.Length; i++) {
byte[i] = String.Asc(Characters[i]);
}