How long has she been looking "skinny"? I find all my horses loose weight in the summer months...not alot, but enough to trim up a it.
First off they all are quite fat from winter of not doing as much and eating a lot more to stay warm.
Also I find that in summer w the bugs n heat, and increased excersice they do drop a bit of weight,
Now this inst bad in my case, as I say its jst more or less thier winter weight so to speak....nothing that would send alarm bells.
However some horses can loose alot of weight in "bug season" and may need a bit more calories to help keep an adequate weight.
If you're concerned about weight...adding some beet pulp to her diet is helpful, and a good supplement for her coat is ground flax.
Now if you're just wanting to add vitamins and minerals "just because", then throwing a trace mineral block into her paddock or stall is the easiest way of accomplishing that.
Horses are like chips...you can never just have one