see steps here - the contribution is backdated to prior tax year but the conversion is reported in the calendar year it is made once you have the 1099-R. The contribution will be reflected on Form 8606 Line 1 and 14 which will carry over to your 2026 8606 Line 2 as the basis for the conversion you would report in the 2026 tax year once you have the 1099-R
https://ttlc.intuit.com/turbotax-support/en-us/help-article/retirement-benefits/enter-backdoor-roth-...
