It's very interesting that while all IRS FIRPTA withholding payment literature discuss making a payment the publications, website and other material never tell you just how to do that and only merely say it's due with Form 8288.

However, IRS Pub 4990 ELECTRONIC FEDERAL TAX PAYMENT SYSTEM PAYMENT INSTRUCTION BOOKLET FOR BUSINESS & INDIVIDUAL TAXPAYERS does say that Electronic Federal Tax Payment System (EFTPS) can be used for 8288 payments.

Valid Tax Period Ending Dates to use in said system when making 8288 payment: 01–12

The Financial Institution Tax Form Code No to use when making 8288 payment is:

82881 when payment is due with an 8288 return

82887 when payment is due on an IRS notice