After you file

You can choose to apply a portion of your 2020 refund to the next year's tax bill.  You just need to enter an amount instead of percentage.  

In TurboTax online,

• Sign into your account, select Pick up where you left off
• From the top, select Other Tax Situations
• Under" Additional Tax Payments" / Apply Refund to Next Year, select Revisit
• Next screen, "Apply How Much?", enter the amount.  See image