You pay it seperately. You don't report it anywhere. The deduction for this deferral amount was already calculated back in 2020. On your 2020 tax return the full amount of self-employment tax, before the deferral, is on Schedule SE line 12 and Schedule 2 line 4. The deduction for half of that amount is on Schedule SE line 13 and Schedule line 14. The amount that you deferred is on Schedule 3 line 12e.
See HERE for more information.
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