Follow the tabs at the top in TurboTax Desktop. Make sure you are on the FEDERAL TAXES tab, then navigate to the Other Tax Situations "sub-tab". Once there, go down to the Other Return Info section, and you will see the Identity Protection PIN section. Work through the interview questions to enter your IP PIN. Here's a helpful link that also explains how to enter your IP PIN in TurboTax Desktop; How do I add or remove my 6-digit IP PIN?