Enter the Form 1099-R exactly as received under Wages & Income -> Retirement Plans and Social Security -> IRA, 401(k), Pension Plan Withdrawals (1099-R). After doing so and answering the immediate follow-up questions to indicat that you "did something else with the money" (i.e., you did not roll it over), click the Continue button on the Your 1099-R Entries page and proceed to the page where you can and enter the amount in the Equal Periodic Payments box. You can also reach this page by going to Other Tax Situations -> Additional Tax Payments -> Extra tax on early retirement withdrawals.