It has to be hard wired into the central locking system. Ie. a pair of wires run to one of the existing solenoids. The canbus doesn't know how many solenoids are fitted as different specs and a topbox can vary the number, so it just works.
If you post an email address I will send you photos and a more detailed description of how to do it.
Regarding BMW doing it, I think it may have been originally intended and then not. Perhaps another solenoid would have put it over budget. Gone are the days when engineers designed it and the accountants then set a price. Which is probably why they ended up with a recall for the suspension. Accountant won the argument and the cheapest was fitted.