I don’t have a cam but have two other Psalmopoeus and keep the sub slightly moist for slings and small juveniles by slightly overfilling the water dish. I do let it dry out in between though to reduce chances of mold creeping up. Once larger they won’t require more than just a filled water dish. Pretty hardy and low maintenance in my experience.
The substrate should not dry out completely, as one likes to do with other species. Real plants are a good indicator here. As long as they are doing well, there is usually sufficient moisture...
If you don't want to constantly monitor the supply, just put a little more plants and moss in the enclosure. There is always some moisture left.
I always keep mine with some moisture in the substrate since it doesn’t take long at all to do, that I allow to dry out occasionally as they are out of sling stage, but during sling stage I kept them on slightly moist pretty much always.