You enter into 2020 TurboTax the Roth IRA contributions that you made for 2020. By default, contributions are treated as being for the year in which they are made. However, you can explicitly designate to the IRA custodian that a contribution made between January 1 and April 15 is to be a contribution for the previous year. If you are unsure as to the year for which a particular IRA contribution was designated, ask the IRA custodian. If you find that a contribution made between January 1, 2020 and April 15, 2020 has been designated as being for the wrong year (2021 instead of 2020 or 2020 instead of 2021) you have until April 15 2021 to ask the IRA custodian to make the correction.
Use the actual contribution date for your Roth IRA contribution for Tax Year 2020.
You are allowed to designate a Roth IRA contribution made through April 15, 2021 as a 2020 contribution.
For more information, see this section from Publication 590-A, Contributions to Individual Retirement Arrangements (IRAs): When Can You Make Contributions?