I would like to know which product to use for filing as "Mark to Market" Active stock day trader. I believe the forms to use would be IRS form 4797 and 6781?

You can use either the Self-Employed in TurboTax Online or the Home & Business TurboTax Desktop for your activities as a "mark-to-market Active stock day trader.

Where do I select the cost basis Mark to the Market for tax year 2021 on form 4797?

When you file your first securities business return, you must elect an accounting period. On the Form 3115 filed for the year of change, a taxpayer should indicate that the taxpayer has filed the statement in compliance with section 5.03(1) of Rev. Proc. 99-17. See Form 3115 instructions page 4 right hand side for mark to market. You can file it with your return.

@mccooney