You do not need to know your MAGI to open and contribute to a traditional IRA. Contributing to a traditional IRA only requires that you have sufficient compensation on which to base the IRA contribution.
What does depend on your MAGI is your eligibility to claim a deduction for the traditional IRA contribution when you or your spouse is covered by a workplace retirement plan. If your MAGI and workplace retirement-plan coverage make you ineligible to take a deduction for the contribution, the contribution must be reported on Form 8606 as a nondeductible contribution.
If you have entered a traditional IRA contribution, TurboTax shows your MAGI on line 12 of TurboTax's IRA Deduction Worksheet ("IRA Worksheet" in the forms list). Note that line 11 of this worksheet has not yet been updated to show the 2020 MAGI thresholds, so until the developers update those thresholds from the 2019 amounts you'll need to override line 11 and replace the 2019 amount with the 2020 amount.
