As a distribution of prior contributions is not taxable, you'll need to enter your prior years' Roth IRA contributions.
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.
To be clear, although this distribution is a nontaxable distribution of Roth IRA contribution basis, this distribution is not a qualified distribution because you are under age 59½ (and presumably not disabled). Nonqualified distributions such as this are required to be reported on Form 8606 Part III where your Roth IRA contribution basis will be subtracted from the distribution amount to determine that the distribution is nontaxable. Enter you Roth IRA contribution basis as MinhT1 described so that TurboTax will have this information needed to complete Form 8606.