How much did you read or search yourself?
Adding to strings or putting string together is called 'concatenation' in case you need it again. A simple 'STX character' in google gave me the hex value. (An elaboration on the fact that this is a special code could have been helpful, I first thought you meant to add a tag "<STX>")
To answer your question.
I think the following should do the trick or at least get you on the way:
int stxval = 0x02;
string mystring;
StringBuilder builder = new StringBuilder(mystring);
mystring.Append(stxval);
I found the hex value
here[
^]
hope this helps.