To add your domestic partner to your benefits, the carrier requires a copy of the domestic partnership certificate.
Changes can always be made during open enrollment.
If you are outside of open enrollment, you must apply within 30-days of a "qualifying event."
A qualifying event definition can be found here:
https://www.healthcare.gov/glossary/qualifying-life-event/
We can add your domestic partner to your benefits (medical/dental/vision etc), first of the following month.