You will enter it in the Other Tax Situations section of TurboTax.
- Log back into your account
- Click Take Me to My return
- Select Federal Taxes
- Then, select the subtab of Other Tax Situations
- Scroll down to the Other Return Info section
- Click start next to Identity Protection Pin (see attached screenshot)
You can also use the search box to get to this section. Log into your account and select Take Me to My Return (you must click this button before searching or you won't get the jump to link). Click the search icon in the upper right corner (magnifying glass) and enter "IP Pin" in the search box and click the "jump to IP Pin" link that appears in the search results. This will take you directly to the appropriate section.