Try different types of salts
Maybe a lick block or mix some salt supplement into Apple sauce or something like that or you could make treats with the supplement that might work.
I add an ounce of salt in my horses feed everyday ( 1 small scoop) -just regular salt that you can get in a huge bag at Costco does the trick. In the Summer or when they sweat more I will put in an electrolyte also, like Apple A Day by finish line.
Try regular salt and mix it with apple sauce (plain) and mix in with his/her food. I've heard of this being done where I board.
If your horse is refusing mineral blocks then it generally means they don't need the minerals in it. However, if you're still worried about giving them the minerals they need, try a supplement powder of pellet to mix in with their grain.
I would check your feed and see how much salt content is in it. Then possibly feed a granule portion of salt mixed in with her feed.
We have a horse who is a bit of a worrier and always sweats excessively in rides, so (aside from her salt lick) we give her salt supplements every day, because she looses so much, and isn't all that interested in salt licks. She doesn't really like the flavour, so what we do is not give her too too much so that she doesn't leave a bunch behind, and sometimes wet her feed so it sticks.
Salt is the one mineral that animals will self regulate; if their bodies need salt they will lick a block.
There are situations in which a horse should be given extra salt, a horse that gets impaction colic or a horse with anhidrosis would be two- but it should be with a vet's supervision.
Most processed horse feeds already contain salt, so your horses are probably getting what they need from those :)
My mare will not lick a salt block either. I have tried regular salt, himalayan, and mineral salt. No deal. The feed store near me has a granule form of the mineral salt block so she gets this in her feed each day.