If you filed the return already and are just now realizing this - then you will have to wait for the IRS to reject your return before you can fix it.
Otherwise, you can enter/fix it using these steps -
Log into your account and click Take Me To My Return (must click this or will not get the jump to link in next step). 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.
You can also get there through the Other Tax Situations section of TurboTax.
Log back into your account
Click either Take Me to My return or you may see an option to Fix your Rejected 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
