Some horse don't like to lick salt blocks, while others do.
It won't hurt to add loose salt to their feed pans as long as they have a bucket of water in their stall. Regular table salt works and just for the record Kosher salt is excellent. I use Kosher salt on new hay, when we stack it in the barn, to help wick any moisture.
A horse should have 10 grams of salt daily; that equals two tablespoons.
You can start out one teaspoon at a time, working your way up to at least one tablespoon daily. If the horse develops loose manure, it's getting too much; there is some salt in hay