If you have not yet filed the 2018 tax return you can also apply the 2018 tax return to the 2019 taxes.
To add, change or delete a current year federal tax refund to next year's taxes before you file your tax return -
Or enter apply federal refund to next year in the Search box located in the upper right of the program screen. Click on Jump to apply federal refund to next year
The amount applied to next years taxes will be shown on Form 1040 Schedule 5 Line 66 and flows to Form 1040 Line 17