IRS direct pay allows you to make various payments to the IRS.
If you are trying to make estimated payments, please note you can only do so until January 15 of the year for which you're making estimated payments.
If you have unpaid vouchers after January 15 and want to pay the IRS for your upcoming tax return, please select Form 1040 as your payment option even if you are paying the estimated payment amount. The most important thing is to ensure that the payments are applied to the correct year.
Select 'Estimated Tax' as your reason for payment and the tax year you will be paying towards. Unlike payment towards a prior tax year, estimated payments are for the future, so you are likely paying toward the current year.
As a rule of thumb, if you owed tax last year, you should make estimated payments of 110% of your prior year's tax due to avoid underpayment penalties.
Please see the sample image below.
Please see this article for more information on how to pay the IRS,