I want to know what form to use on turbotax to make Mark-to-Market election for next year.

To make the mark-to-market election for 2017 you need to attach a statement to your 2016 tax return.  There is not an actual form for making this election.  In order to make this election you will need to print and mail your income tax return in 2016.  The statement should include the following information:

1. That you're making an election under section 475(f);
2. The first tax year for which the election is effective; and
3. The trade or business for which you're making the election.

When you file in 2017 you will need the online Self-Employed or cd/download Home and Business version of TurboTax since your trading will be reported as a business.  

Please see Tax Topic 429 for more detailed information on how to make the election in 2016 effective for tax year 2017, how to report this on your 2017return, and special requirements to be able to make this election.

https://www.irs.gov/taxtopics/tc429.html

There IS an actual form for making this election. It is Form 3115.
DCN64 (DCN stands for “Designated automatic accounting method change") is used to make this election.
Form 3115 can be found at:
<a rel="nofollow" target="_blank" href="https://www.irs.gov/pub/irs-pdf/i3115.pdf">https://www.irs.gov/pub/irs-pdf/i3115.pdf</a>

There IS an actual form for making this election. It is Form 3115.
DCN64 (DCN stands for “Designated automatic accounting method change") is used to make this election.
Form 3115 can be found at:
<a rel="nofollow" target="_blank" href="https://www.irs.gov/pub/irs-pdf/i3115.pdf">https://www.irs.gov/pub/irs-pdf/i3115.pdf</a>

Form 3115 is not for making the election. You make the election by attaching a statement to the previous year's filed taxes. Form 3115 is used with the filings of the first year that mark-to market is used.

Example:

April 2019: When filing your 2018 taxes (by the original Apr. 15 due date) you attach a statement which includes all of the information in the original answer.

April 2020: When filing your 2019 taxes, you include form 3115 to initiate the new accounting method. (This form is only needed for the first year of mark-to-market accounting.)

Just a comment:  Why doesn't Turbo Tax allow for appending documents or plain text statements to the form? It would certainly be more convenient and/or secure than having to print everything out, make our own document for the statement, then mail everything to the IRS...

Thank you for your very informative email.  Do you know whether TurboTax supports the MTM accounting method for trading after the MTM election is made?  I made the election for the 2018 tax year outside of TurboTax but would like to return to TurboTax this year.  Thank you for your help!

TurboTax does not support the MTM process. You can download the form from IRS. 

See additional information in the thread below:

Mark-to-Market Election