How to input a non-estimated pre-filing payment? Paid online as a 1040 payment (not 1040ES) for 2018 in March. Line 66? Can't edit line 74?

I don't want the IRS to be confused by a mismatch if I put it on line 66 as an estimated tax payment, because it wasn't submitted with a 1040ES.  I think technically it should be on line 74, but I can't figure out how to edit line 74.