Greetings!
Here is what I read, this is from a US Embassy from a different country, but I am pretty sure that this is a standard, however if you want to double check, please contact your embassy.
reference: Ask the Consul - Adding Derivatives or FTJs - U.S. Embassy in the Dominican Republic (usembassy.gov)
Did you know? You can add a “derivative” to your immigrant visa petition just by accessing your CEAC application! You will need all of their information, including first and last name, method of contact, and relationship to the principal applicant. Keep in mind that a derivative can be either a spouse or a child younger than 21 years of the principal applicant. You can make this change from the case summary page’s status chart. Go to the list of visa applicants under “Applicant Information”, click on the drop down box and select either “follow-to-join” if you’d like the derivative to travel later, or “accompanying” if you’d like the derivative to travel with you. The change will be reflected one or two hours after making the selection.
Don’t forget! Once you have made your selection, fill out the required application forms, obtain the required civil documents, pay the required fees, undergo the required medical examinations, and schedule your appointment at the Visa Applicant Center.
If you want to add a derivative but already have your Embassy interview scheduled, please let the Embassy know before the interview – our contact information is on the paperwork you received. You will still need to complete all required processes – their own application, civil documents, medical exam, VAC visit, and pay all required fees. Keep in mind that if you’d like your child to travel with you, they need to be present at your interview!