Screen 25 > Taxes > Put a 2 in the first box. This will check the box on 5a (If there is sales tax). If you do not have a sales tax, scroll down a bit and put a 1 in for Actual Taxes Paid (Let's assume you crossed the border and paid some tax in a neighboring state (or make up your own story))

https://accountants-community.intuit.com/articles/1608009-state-and-local-sales-tax-deduction