Short answer: Submit an enrollment/change form to the carrier within the allowed window; within 30 to 60 days of a qualifying event like marriage or a new child, or during open enrollment, with any required documentation.
Adding a dependent to a dental (or any) plan is done through the carrier’s enrollment-change process. Outside of open enrollment, you generally need a qualifying life event; marriage, birth, adoption, or loss of other coverage, and you must submit the change within the plan’s special-enrollment window (typically 30 to 60 days), along with any supporting documents. Coverage then takes effect per the plan’s rules. Missing the window usually means waiting until the next open enrollment.