Yes, since the locality is the same for Box 20, you can add the amounts in Box 18 and 19 together and enter the total.
TurboTax does not automatically fill out a city return unless the taxpayer initiates it in the State section of the software. After the taxpayer goes through the city questions they would need to print and mail in the return to their local municipality.
If you did not complete this section, you can go to your City's website where you need to file, and complete an online return. There are cities who will prepare your return in person for free, contact your City's Taxation Department for more information.
Here's more info on City Tax Returns.
@Demarie
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