Bernie is right on; you can configure the "first and second Append Characters" in the Channel 1 General Configuration window for the MicroLogix 1500 in RSLogix 500.
This only affects the characters that are appended to the end of a string that is transmitted from the serial port using the AWA instruction (ASCII Write with Append).
There's another setting on the Channel 1 configuration window, for string Termination Characters. These are defined for incoming strings, and are usually CR and LF. This tells the controller what to look for to consider an incoming string of characters a "line" for the ARL (ASCII Read Line) serial port instruction.
Your application is different from both of these; you need to pre-pend as well as ap-pend characters to your string.
I would go with Bernie's strategy of simply entering them manually into the string contents. You can specify unprintable characters with their control codes (by typing "^B" or "^C" where the caret "^" is shift-6, not Control) or with a backslash and their hex value; "\02" and "\03".
Your string entries then become like this:
^B123OutputString^C
or
\02StringForOutput\03
RSLogix 500 will always display your unprintable characters using the first method, so I usually type them in as hex and then watch them turn into Control Characters to remind myself I got the correct ones.