The solution is not readily apparent, but here's what worked for me using Turbotax 2024
When working in the step-by-step view for Retirement plans, select the OPM 1099-R entry under your list of 1099-R entries. The series of screens will ask to confirm payer info, enter annuity info, ask about you being a public safety officer (no), box 5, if it's a 403(b) plan (no), about RMDs, what you did with the money, and if you got regular payments (yes), and if those started this year (no).
The key question is next - was the taxable annuity amount used as the taxable amount. Well, since the OPM form says "UNKNOWN", one could assume it was the same amount. But chose "No, a different amount was taxable". That allows you to go on to screens allowing you to use either the IRS simplified method or general rule (I chose simplified), followed by other questions that allow Turbotax to do the calculation.
