if you didn't file 8606 in 2024 and just need to record the basis you don't need to file an amended return you just need to send in the 8606 so it may be easier just to do it by hand. There will be a question in 2025 you probably found already, for you to input the basis to carry forward from 2024.
see "If your conversion contains contributions made in 2025 for 2024" here:
https://ttlc.intuit.com/turbotax-support/en-us/help-article/retirement-benefits/enter-backdoor-roth-...
