5 CFR §875.213

You may apply for coverage as a qualified relative if you are a domestic partner, as described in § 875.101. As prescribed by OPM, you will be required to provide documentation to demonstrate that you meet these requirements, and you must submit to full underwriting requirements. However, as explained in § 875.210, if you lose your status as a domestic partner, and therefore status as a qualified relative, before your coverage goes into effect, you are no longer eligible for FLTCIP coverage.