Federal Income Tax Additional Withholding

Filled 2022 taxes and received a return. Received a letter from the IRS saying now we owe. When reviewing our tax form, I don’t see the FIT additional withholding applied. Our W-2 does not include the FIT additional withholding either. Does this need to be manually entered on the 1040 form and if so, which line? Would we have to file an addendum?