OK, here's how to make the 2018 Legacy OEM fog light kit work on the 2019 model.
As with any electrical work, be sure to DISCONNECT THE NEGATIVE BATTERY CABLE before starting!
Follow the instructions that are readily available online (but they don't come with the kit.) The wiring for the lights themselves is there and the relay and fuses were already installed.
When you get to the switch, there is NO "prearranged connector" for it. I searched for hours for it and the dealer couldn't find it either. They simply told me that the kit wouldn't work on the 2019 and there was no "correct" kit available for it.
Not being one to accept defeat so easily, I bought the service manual (in .pdf form) and studied the wiring diagrams. I found that the wiring for the lower-dash switch (if it's there) simply parallels the wiring for the ring type switch on the signal / headlight switch stalk. I pulled off the lower steering column shroud and sure enough, the wires for the fog switch were there. I jumped across them and the fog lights came on and worked just as designed. On with the low beams (auto or manual mode) off with the high beams. The green indicator next to the speedometer lights up when the fogs are on.
I used a "fox and hound" cable tracer to find the white wire from the switch. It reappears in a black, 32-pin connector on the rear of the under-dash fuse & relay panel, on the bottom right looking from the driver's seat. The wire is white, at the bottom of the connector next to a yellow wire.
I used a posi-tap (I HATE the Scotch Lock type taps!) and brought a wire to the area of the switch. The other 3 connections needed for the fog switch can be tapped off the trunk opener button. The kit comes with a short pigtail that connects between the fog switch and the missing harness connector. Cut off the harness end and connect the wires from the switch as follows:
White to the wire you brought from the back of the fuse panel (to relay);
Blue to the Black wire on the trunk button (switch ground);
Red to the Green wire on the trunk button (switch backlight hot);
Black to the Brown wire on the trunk button (backlight ground).
The instructions say to install the fog switch in the bottom row, far right of the lower-dash switch panel, but that position was taken by one of the Eye Sight buttons on my car. The second spot from the left was empty so I put it there.
My car is a 2019 (U.S.) Legacy 2.5i Premium with the all weather package and Eye Sight. Other models should be similar, but your mileage may vary.