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.
You will then be able to go in and make the necessary changes to the relevant benefit.