It appears you are referring to “postponed retirement” rather than “deferred retirement”. With postponed retirement, you resign and leave your FERS contributions on deposit with OPM at a time when you are already eligible to retire (e.g., you’re your MRA with at least 10 years of service). With deferred retirement, you do the same, but you are not eligible to retire.
The two types of retirement have some similarities: 1) You are not eligible for the supplement with either one. 2) You use the same form to apply for both.
The form is the RI 92-19 and it can be found on the OPM website. About 60 days before you want your FERS annuity to begin, send the form to: OPM, FERS, PO Box 45, Boyers, PA 16017-0045.