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...

The IRS will be adding that to their portal soon.

Go to this IRS website for information - https://www.irs.gov/newsroom/irs-online-tool-helps-families-see-if-they-qualify-for-the-child-tax-cr...

Child Tax Credit Update Portal

Earlier this week, Treasury and IRS launched another useful tool, the Child Tax Credit Update Portal. Initially, this tool only enables anyone who has been determined to be eligible for advance payments to see that they are eligible and unenroll from (opt out of) the advance payment program. Later, it will allow people to check on the status of their payments and make updates to their information, including their bank account information. Later this year, the tool will also be available in Spanish.

For more information, visit IRS.gov/childtaxcredit2021, or read FAQs on the 2021 Child Tax Credit and Advance Child Tax Credit Payments.

Later this year, the Child Tax Credit Update Portal (CTC UP) will be updated to allow you to inform us about the qualifying children you will claim on your 2021 tax return so that we can adjust your estimated 2021 Child Tax Credit – and therefore adjust the amount of your monthly advance Child Tax Credit payments.