No, you do not have to use another tax form. Simply enter all of the information as is on the screen in the system - including the amount in box 2a, then on the next screen titled "Let us know a little more about this distribution" - since the funds were after-tax dollars - check off the circle to the left of "Roth IRA" and at the bottom check the circle to the left of "No" if no penalty applies to you for this withdrawal. You've now told the system that the IRA contributions were made with after tax dollars and this end result will be a nontaxable distribution.
