The guidance on where to enter this information sits within the IRS Letter 6419.

Box 1. Aggregate amount of AdvCTC payments you received for 2021.
Enter this amount on Schedule 8812, line 14f or line 15e, whichever applies.
If you file a joint return for tax year 2021, you must add the amounts in Box 1 from both
Letters 6419 and enter the total amount on Schedule 8812.

Box 2. Number of qualifying children taken into account in determining the Advanced CTC.
See Schedule 8812 instructions if you complete Part III, Additional Tax.

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