The child tax credit will be added to your return when you input your child's information in your 2021 taxes. You don't have to fill out any particular form - just put in your child's information (full name, date of birth, and social security number, etc.), and the credit will be applied to your return.
The Advance Child Tax Credit - monthly payments that are received starting in July of 2021 - can be applied for or modified at the following portal: https://www.irs.gov/credits-deductions/child-tax-credit-update-portal
These payments will be an offset to the Child Tax credit received with your 2021 tax return.
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