For 2019, you will simply enter the IRA contribution. Deductions > Adjustments to Income > IRA Contributions (1=maximum deduction) > Enter the amount the client intends to roll over next year. Also enter Contributions made (letter use only)

That opens the back door, but that door can't closed until 2020, when it is withdrawn from the IRA and rolled into a Roth. 

Careful- Are there any other IRA accounts? 

Thank you for the response. So even though these transactions happened almost simultaneously (IRA contribution and conversion to Roth), the conversion is considered to have occurred in the actual year of the conversion (2020). Correct?

Yes. Contribute for 2019 in 2020 goes on 2019. Conversion for 2020 in 2020 goes on 2020.