Withdrawals of your contributions to a Roth IRA are not taxable. In TurboTax Online, you'll have to enter your prior year Roth contributions so that TurboTax knows not to tax your withdrawals.
After you have entered your form 1099-R, TurboTax will take you back to the summary page titled Your 1099-R Entries.
On that page, click Continue and follow the TurboTax interview until you arrive at the page titled Enter Prior Year Roth IRA Contributions where you can enter the amount of your past contributions.