TurboTax does not support the MTM process. You can download the form from IRS.
https://taxmap.irs.gov/taxmap/ts0/howtomakethemarkto_o_6170beaa.htm
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.
If you've made a valid election under section 475(f), the only way to stop using mark-to-market accounting for securities is to file an automatic request for revocation under Revenue Procedure 2018-31, Section 24.02. Under that revenue procedure, the request for revocation must be filed by the original due date of the return (without regard to extensions) for the taxable year preceding the year of change (the year of change is the first taxable year the revocation is to be effective). This revocation notification statement must be attached to either that return or if applicable, to a request for extension of time to file that return. Late revocations won't generally be allowed except in unusual and compelling circumstances.
