contributed 13500 to IRA during 2024. (6500 from 2023, 7000 from 2024). Then did a backdoor conversion. Now I'm struggling to show that those 6500 belong to 2023.

I know something is wrong because my tax due increases, that shouldn't happen in backdoor conversions (when triggering form 8606). Thing is, I can only trigger it for 2024. How to do it for 2023? (without any tax changes in my 2024 declaration)