Option 1: smart switches
In this case you locate and obtain a "smart switch" transmitter/receiver combo, where the transmitter sits in a wired switch location and the receiver sits up in the fan housing. The switch and receiver must communicate wirelessly.
Then you re-task the black wire in your switch run to be always hot for all of these loads: the smart switch, the reciever, and the automatic humdity sensor.
Option 2: you are lucky and the power goes to the fan first
It sounds like you don't... But if the power comes initially to the fan housing (2 Romex's there and only one in the switch box) then the fan housing already contains everything you need. The supply gives ground, neutral and always-hot. The switch loop provides your switched-hot. You are all set.
(Though I don't think you'd be here if that was the case).
Option 3: You don't.
Aside from the smart-switch method I described, you are out of luck. You cannot bring always-hot and also switched-hot using a single black wire. White must be neutral and bare must be ground. Cheating ground in some way is a recipe for disaster in a bathroom. Even a GFCI does not make this safe.