Since the payment was made for the 4th quarter of 2018, it has to be reported on your 2018 return. It cannot be reported on your 2019 return.
If you have already filed your 2018 return, you can amend it and include the payment. Please see the instructions contained in this TurboTax Help article to amend your return: How to amend (change or correct) a return you already filed
