After you enter your 1099-R, continue through the section. On the screen Any Nondeductible Contributions to your IRA? click yes, you made and tracked nondeductible (after tax) contributions.
You'll then enter your basis (total nondeductible contributions) in the next screen, and the total value of all your traditional IRAs, etc as of December 31, 2017. The taxable portion of your distribution, if any, will be computed based on your entries.
See the screenshot below.
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