If you used TurboTax Online last year, please make sure that you are logged into the same account that you used last year. Then when you start a new 2024 return it will give you a list of 2023 returns to transfer.
If you used TurboTax Desktop last year, then TurboTax Online will ask you what you used, and then give you the opportunity to upload a file after you pass the "Start for Free" page.