I don't think concealing the wire is possible with that model. I use
Lowell's LH-15TA, and you can see in the second picture, if you drill a hole through the blank plate and mount the horn to the blank plate, there is a slot in the mounting bracket to feed the wire.
An alternative would be to use the
LHA-15C conduit adapter, and install a whip of flex from the box into the adapter and secure it to the horn.