you can't pay online like you would an estimated tax payment
you have to register for EFTPS
There’s a system called EFTPS, which is like an electronic funds transfer network that the federal government has. You can access EFTPS here https://www.eftps.gov/eftps/ And so you’re going to use that to pay the federal government. Now remember, you only have to pay 50% of it back in 2021 and 50% in 2022. So you’re going to log in, create an account, and you’re actually going to start sending them money. Now, some companies prefer to do this on a monthly basis, right, so that’s it’s not all hitting at the end of 2021. But other companies are preferring to just wait ‘til late in December and actually just make their 50% payment then. However you do it, it’s a good best practice to establish your account and actually know how it works and be able to fund it. You don’t want to be the people trying to pay this on December 31st, and not be able to figure your account and then not get the money in, and then have a big penalty. And don’t tell our friends at the IRS this, but the government’s software has been known to have technical issues as the deadlines approach as everyone tries to do everything all at the same time…
