You can either leave out loose salt for your horse or add it to their feed. If you add it to the feed, continue to leave out a salt block or loose salt in case more is needed.
Table salt is fine. You can get either iodized or plain/non-iodized table salt, which one you'd pick would ideally depend on how much iodine your horse is getting from other sources, though this can be hard to know since it's rarely included in the guaranteed analysis of feeds. When in doubt, go for the plain salt.
It's generally cheaper to buy a large bag from the feed store than smaller containers from the grocery store, though.
“The horse is a mirror to your soul. Sometimes you might not like what you see. Sometimes you will.” - Buck Brannaman
"Nothing forced can ever be beautiful." - Xenophon