Enter the backup withholding on Screen 22 under Credits, on the line for Other credits (Ctrl+E), and use Code O=Backup Withholding.  It will flow to Line 15 of 1065 Sch K-1.

How about backup withholding on a 1099-K for an S corp?

Same place

The exact location in the S corp module is

Schedule K Section - Screen 26 Credits - Schedule K Credits - Very bottom under other Credits