If you're sure the input value will fit in a single byte, then why are you converting it to an int? You can replace the entire red section with just this:
Code:
OUTBuffer[2] = Convert.ToByte(textBox4.Text);
If it doesn't fit in a single byte, then you'll have to copy all of the bytes instead of just one.
Code:
int decValue = Convert.ToInt32(textBox4.Text);
byte[] intBytes = BItConverter.GetBytes(decValue);
Array.Copy(intBytes, 0, OUTBuffer, 2, intBytes.Length);
And then you'd have to change your for() loop of course. Your comment says you're setting the rest of the bytes to 1, but 0xFF isn't 1...:
Code:
for(int i = 2 + intBytes.Length;i < 65;++i)
OUTBuffer[i] = 0xFF;