see this link and note this key phrase "Should designate the payment as "deferred Social Security tax.""
https://www.irs.gov/newsroom/how-self-employed-individuals-and-household-employers-repay-deferred-so...
from what I was able to check Direct Pay is not available as of yet (can't specify its a deferred Social Security Tax Payment). however, the IRS has until 12/31/021 to update its payment methods. so if you want o pay now that leaves mailing a check or money order or using the EFTPS system which requires you to sign up then wait for a letter from the IRS with a code that will allow you to finish your sign up. while payment by check or money order is possible the IRS does not specify an address to which it should be sent on the link.
