Where do I enter my prior year refund that I applied to this year's state tax?

You enter this in the Federal section.

Go to Federal Taxes

then to Deductions and Credits tab

to "I'll choose what I work on"

to Estimates and Other Taxes paid

to Other Income taxes

to 2017 refund applied to 2018 and enter it here.

Do  Search in the Search box at top of your return for  "refund applied to this year" and then click on jump to link