Has your tax return been accepted yet? If it has been accepted, you can't add that information now. In this case, you will need to wait until the IRS processes your original return. They will most likely adjust your refund to not include the payments you already received. If they don't, you can amend later. The amending software will be available later this month.
If you have not filed yet or your return was rejected, you can go back in and add the payments from your 6419 letter. To do this, search for child tax credit (when you have your return open) and use the Jump to child tax credit link to be taken to the section. Click through the screens and when you come to the question about advance payments, enter the information from your letter. If you have already e-filed, you won't be able to make any changes, see above.
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