It requires 3 identifiers. A Social Security or other taxpayer ID, a PIN that they assign with a letter they postal mail to you, and a password you create. Last Pass only has a user name and password, EFTPS requires 3 entries.
EFTPS also on occasion asks for the enrollment number that was on the letter that the PIN came with.
So I think lastpass should have a special format to hold all that info. EFTPS is a very important website for Americans so LastPass should support it even if it is non standard.
Or perhaps LastPass could help the IRS to use a more modern authentication system.