Turns out it's a trick question.  The answer is: there isn't one!  Dell support says that virtualization is enabled permanently - so no BIOS switch is needed. This can be verified by running task manager, selecting the Performance tab, clicking on the CPU icon and checking the CPU details listed below the graph.   Virtualization will show as Enabled. SO... looks like VMware Workstation 16 isn't ready to deal with the I9 yet.  Thanks to all for the help.   
Hi,  VMware Workstation 15 reports that VTx is not available.  Anyone know how to turn it on?   It's not obvious in the BIOS.  Thanks for any assistance! 
m17 R3, I9 10980HK - can't find BIOS setting for VTx / VTd