Christi's got it right. Or if you don't feel like typing in 1040, you can go to the second drop down menu and select the first option which is for the 1040. The instruction "Please enter the number of the tax form you wish to PAY" is what is confusing. Everybody assumes that it's 1040-ES. It's not, it's for the whole 1040 group.
I think if they removed the word PAY and put in its place "submit payment for." it would be a little clearer. That's how they word it on the Direct Pay Help page. Then when you get to the next page, you have the option of selecting 1040-ES.
Byron
