Solution
The PXIe-408x DMMs no longer have the 9-pin auxiliary (AUX) connector, but they can still receive hardware triggers from the PXI backplane. You can use any PXI module with an available PFI line to import an external trigger and route the trigger to the PXIe-408x DMM along the backplane of the PXI chassis. Example modules are timing and synchronization modules, DAQ devices, oscilloscopes, High-Speed Digital I/O, or any other device with an open PFI line.
The other option available is to use a chassis with a PFI line embedded (REM) in the rear panel like the PXIe-1095 and PXIe-1092 with the Time and Synchronization upgrade.