From a Linux command line it would be the command called arp, you need to add a static arp entry. I don’t know how that works on sense, but on Linux it would be something like
arp -s IP MAC
Maybe there’s a module in opnsense to help. The way I’ve done this before is using a machine connected to the same network at my target to wake up by logging into that machine and issuing the wake command.
I do this with my desktop - I work from home so it’s really nice to have my PC ready by the time I get down to it. There’s a workday integration too, set your typical schedule and it’ll be true when it’s a workday - with a motion sensor as the trigger as my start time varies if I have meetings In the morning.
This is one of the first things I set up with HA for fun but the convenience is really nice.