Maybe it's not the quickest way to do what you require but it's the only way I know how to do it with formulas:
- enter this formula =CONCATENATE(TRANSPOSE(A2:A15)&",") in the cell where you want to join your "TR-CODE"s
- highlight this part of the formula TRANSPOSE(A2:A15)&","
- press F9 key to show the cell contents instead of the cell reference
- from the formula remove the two curly brackets (one at the beginning and one at the end) and the last comma (if you don't need it)
- press the Enter to confirm.