How can I add myself as the tax preparer on a return?

Your tax return is self-prepared using TurboTax as is shown on the printed Form 1040 in the Paid Preparers section

You only list  yourself as the preparer of the return if you are paid to prepare the return. So if you, for example, do a return for a family member for free, you don't list yourself.

If you are being paid to prepare the return you cannot use TT, but you can use their professional software - ProSeries or ProConnect.

Right. OP screen name ends in cpa.

And if paid you may have other obligations with the IRS (continuing education, PTIN registrations), etc. some of which might be waived if you are an attorney, CPA, or EA.