How do I show my early retirement account withdrawals are via a 72T arrangement?That is, I took a distribution in equal periodic payments over my life.

You should have received a 1099-R.   All you need to do is to enter the 1099-R, exactly how it is, and then answer the follow up questions.   TurboTax will correctly report it on your tax return.    Substantially equal periodic payments

Under Section 72(t), there is an additional tax of 10% on distributions to the taxpayer if the distribution is made before the taxpayer is age 59 ½. This applies to distributions from qualified retirement plans, which include:

1. a plan described in Section 401(a) (which includes a trust exempt from tax under Section 501(a)),
2. an annuity plan described in Section 403(a),
3. an annuity contract described in Section 403(b),
4. an individual retirement account (“IRA”) described in Section 408(a), and
5. an individual retirement annuity described in Section 408(b).

The 10% additional tax is determined with respect to that portion of the distribution that is includible in gross income.

if the distributions are determined as a series of substantially equal periodic payments (called a “SoSEPP”) over the taxpayer’s life expectancy (or over the life expectancies of the taxpayer and the taxpayer’s designated beneficiary), the 10% additional tax does not apply.  See requirements in IRS link above.