Click on "where do I enter" then type in election. You get a lot of elections, see it he one you need is there. from IRS:

A trader must make the mark-to-market election by the original due date (not including extensions) of the tax return for the year prior to the year for which the election becomes effective. You can make the election by attaching a statement either to your income tax return if filed without an extension or to a request for an extension of time to file your return. 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 (that is, the tax year for which a timely election is being made); and
3. The trade or business for which you're making the election