The Update Portal for adding a dependent is not available yet. The IRS says it will be available "later this year." If you don't get all the payments for your new child during the year, you will be able to claim the missing amount as a credit on your 2021 tax return.
See Q F3 at the following link on the IRS web site.
2021 Child Tax Credit and Advance Child Tax Credit Payments — Topic F: Updating Your Child Tax Credi...
