26 CFR 16A.126-2: A.126-2 Section 126 elections.

(a) Election for section 126 not to apply in whole or in part. A taxpayer may elect under section 126(c) not to have section 126 apply to all or any part of an improvement described in section 126.

(b) Application of the section 126 exclusion. To the extent the section 126 exclusion applies, the taxpayer should so indicate on an attachment to the tax return (or amended return) for the taxable year in which the taxpayer received the last payment made by a government for the improvement. The attachment should state the dollar amount of the section 126 cost funded by a government payment, the value of the section 126 improvement, and the amount that the taxpayer is excluding under section 126.