It first glance it seems to answer the question but in fact this does not answer the OPs question at all. The IRS notice tells you where to go to make the payment but does not tell you "how" to make the payment. If you go to EFTPS and select "deferred social security tax" as the type of payment, the next screen asks for "month" AND "year" for the tax period. The year is 2020 but what is the month? If you enter December (thinking a self employed person filing form 1040 has a tax period ending Dec 31) the program returns an error. If you leave the month blank, the program also returns an error. You could make an assumption and try some other month but I personally am leary about making asumptions about the correct procedure when it comes to dealing with the IRS. Particularly when the IRS explicitly says the payments "should be a separate payment from other tax payments to ensure they are applied correctly." Then there is the IRS "direct pay" web portal which offers a dozen or so payment options, none of which are designated as "deferred self employment tax." So it seems like the only safe option is to mail a paper check where you can print the words on your check "repayment of 2020 deferred self employment tax".
