Many years ago dairy farming friends used on the herd. We did try tying to horse head collars but not sure they were that effective - I seem to recall that they were intended for use on large groups of animals - maybe a dairy farmer could comment as I am going back some time.
If you go into the "Search" section, at the head of your H&H screen, and type in your thread title, the question came up in June this year, and May last year!
They work very well, but it's vital that they are attached inside a head collar, and left in place. It will take 2 or 3 days, but as they rub against the face of the horse, so the chemical leeches out, and total coverage will be achieved. They work best with one each side of the horse, as coverage seems faster.
Some have attached them on the outside of a headcollar, or tied them to the animals mane. They wont work, they must rub against the horse's face. Left in place they will give 2-3 months protection.