On a 320/30/40 you are the PF taxying the ac and should only have to glance at your ND where you should see TA ONLY in a white box at the bottom. You then reply set.. The PNF will have switched the 3 posn rotary switch to TA/RA as part of his/her pre-takeoff actions.
The code and modes are done at the gate..before engine start, transponder should be in auto mode..