TurboTax does not support the MTM process. You can download the form from IRS.
MTM Election
Refer to the Form 1040, Schedule D Instructions, Capital Gains and Losses for more information on how to make the mark-to-market election. It's important to note that in general, late section 475(f) elections aren't allowed.
After making the election to change to the mark-to-market method of accounting, you must change your method of accounting for securities under Revenue Procedure 2018-31. In addition to making the election, you'll also be required to file a Form 3115.pdf, Application for Change in Accounting Method.
Publication 550 describes the procedures for making an election under the section called "Special Rules for Traders in Securities." Non-filing of the Form 3115 mentioned above won't invalidate a timely and valid election.
