Oddly enough, someone asked that more than 5 years ago.  Not sure the answer is still the same.

https://accountants.intuit.com/community/lacerte-tax-discussions/discussion/adding-direct-pay-option... 

You don't need an account to use IRS Direct Pay.

There are other options/systems that do use an account.

But we need the instructions on how to do that incorporated into the letters since IRS is phasing out accepting paper checks.