After you file
Per the Instructions for Form IT-558:
Complete Form IT-558 and submit it with your return to report any New York State addition and subtraction adjustments required to recompute federal amounts using the rules in place prior to any changes made to the IRC (federal tax law) after March 1, 2020.
Go back to your NYS return to see what updates you need to make under the Changes to Federal Income section as shown on this screenshot:
