The IRS will stop mailing Stimulus checks on 1/15 so if you don't get it by the time the efiling opens at the end of the month you can apply for the missing money on the 2020 return.
The Stimulus checks are really based on your 2020 tax return. Any amount you already got will be reconciled on your 2020 return line 30. If you didn't get a Stimulus Check (and qualify) or you qualify for more it will be added to your 2020 return. But the good news is if they paid you too much they won't take it back.
When you go through the federal review tab it will ask how much you got if any. Or to enter your stimulus payment in TurboTax, type "stimulus" (without the quotes) in the Search box, then click the link that says "Jump to stimulus." If you didn't get one or both enter a 0 and not leave it blank or it won't calculate it.
After you have entered your tax information and are ready for the Federal Review, it will be the first question.
If you are trying to trigger it, you can go to Other Situations, answer any questions and click Done. The next automatic selection is Federal Review and it will ask about payments received.