Hi my dog is A boxer and she is underweight I was wondering what I could feed her to make her gain weight

Well, first you need to be sure she's healthy.
I'm assuming she does eat..
If she's eating and still underweight you need to take her to a vet for a fecal test and any other tests the vet may feel are necessary to diagnose her problem. She could have a whole lot of issues you are unsuspecting of.
This stuff should be your top priority in unraveling the mystery of her skinniness.
Next you should consider the quality of her food.
Message me back with her diet details; what brand and kind, how much, how often, how old she is, etc.

She should be on a high quality food for sure. Ol' Roy and such cheap brands don't cut it, especially if your dog has a high energy level. If it's a cheap brand she has to eat more to get full because most of it is just bulk that she can pass out the other end as opposed to digesting. Foods like Science Diet, IAMS, Eukanuba, and Beneful are highly digestible and you actually get more for your money because she doesn't have to eat as much to get full.
I do not recommend free feeding. You need to read the bag of food, determine how much she needs for her age and weight, and give her that much once in the morning and once in the evening.

She could have worms, so I think the best thing to do is take her to the vet and have them take a stool sample.

Also, the vet will tell you what supplement and how much food she should be receiving.

You know, this could be as simple as "more of what you currently are feeding". Is she still hungry, and will she eat more of the food if you offer it? If so, you might just try upping the amount that you feed her. I do agree with the measuring it out. That way, once you find out what amount works, you can monitor and increase/decrease the amount to match changing energy levels, and see when she's off her food.