You don't add it back. The way it's handled on the New York tax return for 2021 is different because of a change in the way it's treated on the federal tax return. For 2020 the charitable contributions for nonitemizers reduced the AGI on the federal return, so you had to add it back to the "recomputed" federal AGI on line 19a of the New York return. For 2021 the charitable contributions do not reduce federal AGI. On the federal return it's subtracted only after AGI is calculated. So it does not have to be added back to AGI on the New York return. There is no code A-003 on Form IT-558 for 2021.
