Here's the help article:
https://accountants.intuit.com/support/en-us/help-article/retirement-tax-credits-deductions/report-c...
You'll have a nondeductible contribution to the Trad IRA, and that creates Basis for the conversion. Being all and only basis (and leaving $0 FMV in the Trad IRA) is what makes a conversion "backdoor."
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