Any taxpayer entitled to a 2-year carryback under clause (i) from any loss year may elect not to have such clause apply to such loss year. Such election shall be made in such manner as prescribed by the Secretary and shall be made by the due date (including extensions of time) for filing the taxpayer’s return for the taxable year of the net operating loss. Such election, once made for any taxable year, shall be irrevocable for such taxable year.

https://www.law.cornell.edu/uscode/text/26/172