The humidistat is in series from the transformer - in line from the transformer to the solenoid (pic 1 you can see the humidistat top left, exposed wires to right of humidifier (I was testing for current)).
Pic 2 and 3 are the control box - the lighter gauge black wire on top is the one that came loose. The humidifier transformer is not shown, but you can see the neutral and loose wire.
Not sure I understand the L1 suggestion - is this a switched 110 somewhere? Thx, and sorry for the basic Qs, I get by with house wiring, but this is not my forte...