See 1040 instuctions page 104 for schedule 3 line 12e SE tax deferral
https://www.irs.gov/pub/irs-pdf/i1040gi.pdf
Individuals can pay the deferred amount any time on or before the due date. They:
- Can make payments through the Electronic Federal Tax Payment System or by credit or debit card, money order or with a check. https://www.irs.gov/payments
- Should be separate payments from other tax payments to ensure they are applied to the deferred tax balance on the tax year 2020 Form 1040 since IRS systems won't recognize the payment for deferred tax if it is with other tax payments or paid with the current Form 1040.
- Should designate the payment as "deferred Social Security tax."
Individuals making deferred Social Security tax payments in EFTPS should select 1040 US Individual Income Tax Returns and deferred Social Security tax for the type of payment. They must apply the payment to the 2020 tax year where they deferred the payment.
