There are zero ways to use this cord correctly. Do not tell people otherwise. The proper generator connection is to, as you just said, use a proper generator inlet and interlock like you said you use. Anyone using a cord like this is using it incorrectly as there is no correct way to use it. Both the CEC and the NEC expressly forbid these. All it takes is for a child to unplug the wrong end and touch the exposed prongs and they will get shocked.

A proper generator inlet should have the male ends in a recessed inlet box connected to either a transfer switch or an interlocked generator panel.