Enter the Form 1099-R exactly as received. Do not modify it by entering anything in box 2a or box 5.
After entering your Forms 1099-R, click the Continue button on the Your 1099-R Entries page and proceed to where TurboTax asks you to enter or confirm your basis in Roth IRA contributions. TurboTax will use this to prepare Form 8606 Part III to calculate the (presumably zero) taxable amount of your Roth IRA distribution.
