Most benefits have an enrolment window which, while open, will give you the ability to Opt in, Opt out and/or add dependants to your policy.
If your benefit has an enrolment window which has now closed, you will need to wait for the enrolment window to re-open in order to make any changes, such as adding a dependant or even opting out.
However, if you have recently experienced a qualifying life event, you can contact your HR admin who should be able to open the window for you on your account or submit a Qualifying Life Event request if this is enabled for your account.
You will then be able to and make the necessary changes to the relevant benefit.
Please note: it's not always possible to re-open an enrolment window and this needs to be evaluated on a case-by-case basis.