You have to enter the amount that you applied from your 2018 state tax return in the federal section of TurboTax. In the Search box type "applied overpayment" (without the quotes), then click the link that says "Jump to applied overpayment." That will take you to a screen that says "Estimates and Other Income Taxes Paid." Scroll down to the section "2018 Refund Applied to 2019" and click the Start button for "2018 refund applied to 2019 state taxes."
If you transferred information from your 2018 tax return when you started your 2019 tax return, you might find that the amount has already been entered.
@fanfare I didn't say you could apply the state refund to federal tax. But the state refund applied to state tax has to be entered in the federal interview in TurboTax. A federal refund applied to federal tax and a state refund applied to state tax are both entered under Other Income Taxes > 2018 Refund Applied to 2019. Under that heading there are three separate topics for federal, state, and local. The state screen asks what state it's for. There is no place to enter it in the state interview.