It may not be there to be found, @antonical. Even on older series Samsungs, it doesn’t show up in the NASA traffic on F1/F2. I haven’t tried looking at F3/F4, though the WRC - which does show the offset at least on non-integrated units - looks there all the time of course.
@toadhall gave you the Modbus addresses (Samsung HTQuiet 8KW Idle power consumption 60W? - #16 by toadhall) and I don’t see the offset there either.
I surmise that the controller creates internal alternate registers for #2021/2 based on the FRC offset value and uses these rather than the true #2021/2 registers for WL target calculations. (At least that’s what I would do if I was a controller programmer which, happily for other Samsung owners, I definitely am not 
).
So you may have no option but to manipulate FSV#2021/2 from HA. This is dead easy on Gen6/HTQ series of course (see above posts) but maybe not on a Gen7.
(You may have more luck if you ditch Modbus/MIM-B19N altogether in favour of simple NASA via F1/F2. Without first hand Gen7 experience I can’t guarantee success, but I’d definitely give it a try, particularly if you already have the RS485-Ethernet adapter to hand.)