I missed the last estimated tax for 2020, where to pay then?

I haven't done this myself, but the advice I have heard is choose 'Extension' as the reason, and apply payment to Form 4868. The reason is because there is a specific spot to list this payment (Form 1040, Schedule 3, line 9). You're automatically granted the 6-month extension, but you're under no obligation to use it.

Of course, as others have said, we're talking about 3 percent interest for a few months, which is likely to be minimal. But if you want to make a payment, that's how I'd do it.