How do I report the RMD from my Prudential 403B. They don't send me a 1099R, just a so-called form5498 which TurboTax doesn't tell me how to use it?

You have to get a 1099R form.  The 5498 should only show contributions not withdrawals or it might show the RMD you will need to take for next year.  Did you get the RMD distribution?  If you got a check maybe the 1099R was part of the check stub.  Or maybe you can log into your Prudential account and download another copy.