After you file

Federal Taxes Tab or Personal (Home & Business)
Other Tax Situations
Additional Tax Payments
Apply Refund to Next Year - Click the Start or Update button

To apply your state refund to next year, enter it under
State Taxes tab
And keep continuing and stepping though the interview about a dozen or more screens and it will finally ask how much you want to apply.  It is probably one of the last questions in the state section.
https://ttlc.intuit.com/questions/1901277-applying-your-tax-refund-to-next-year-s-tax-return

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