Quick search by citation:

5 U.S. Code § 8436 - Administrative provisions

The Executive Director shall make or provide for payments and transfers in accordance with an election of an employee or Member under section 8433 or 8434(b) of this title or, if applicable, in accordance with section 8435 of this title.
Any election, change of election, or modification of a deferred annuity commencement date made under this subchapter shall be in writing and shall be filed with the Executive Director in accordance with regulations prescribed by the Executive Director.