do you need to put moisture into the subflooring before installing the thinset under the hardibacker? thanks
No you don't.
Before you install your Hardibacker over your plywood subfloor the plywood should be structurally sound and clean.
We scratch up the plywood with a wire brush, vacum and use a damp sponge to clean it up. The plywood is far from wet but it is clean and free of any bond breakers.
Insure you apply thin-set and then drop the hardibacker into your wet thin set quickly and nail it down. Screws if you prefer.
Do not nail the perimeter and then the center. Start along one long edge and work out from there. Like you are working air bubbles out from under wall paper. This way there is no chance of having the board crowned and flex in the center.