I filed my tax return but I found out I need to submit a T1198 form becaue I have a qualifying retroactive lumb-sum payment

This TurboTax Canada FAQ tells you how you can do that using ReFile: I've filed my tax return. Can I still make changes?

To enter your T1198 (Statement of qualifying retroactive lump-sum payment) in TurboTax Online, search for “T1198” on the Tax Home page (2024 tax return on the left), and select “T1198” from the list.