Go to Deductions & Credits -> Retirement and Investments -> Traditional and Roth IRA Contributions, indicate that you have a traditional IRA, then proceed to where TurboTax asks if you made any nondeductible traditional IRA contributions for years prior to 2019. Answer Yes, then enter the basis that you carried into 2019.
Also, if you haven't started your 2019 tax return yet (or you choose to start over), you can still upload your 2018 TurboTax tax file and transfer it in, if you are beginning from your own tax file rather than your new spouse's tax file. That should carry your basis into 2019 TurboTax Online: https://ttlc.intuit.com/community/entering-importing/help/how-do-i-transfer-last-year-s-return-into-...
