How many is a "few" ribs?
Bear in mind that on a boxer of correct weight, there should be a clearly defined waist (midsection) you should always be able to see the last couple of sets of ribs. If you cannot see the outline of those ribs when the dog is just standing around normally (i.e. she should NOT be stretched out for you to see them), then the dog is actually overweight
A boxer is a
lean breed - more akin to greyhound (muscles on frame) than labrador (inches of flesh covering frame). 80% of pet animals may fall into the latter category, but then, 80% of pet animals are overweight - some dangerously so. Your description of solid muscular front, clearly defined midsection and a few ribs visible sounds suspiciously like a dog of perfect weight to me... though, per above question, we may well have a different idea of what constitutes a "few" ribs
Unless your dog is seriously underweight, I would be more inclined to let her fill out naturally. At 15 months, she's still far from an adult - most boxers are between 2-3 years old before they have fully developed their musculature and given up the lanky look.
If you really do feel that you have to do something to help her gain weight - don't do it by piling fat into her diet. That is not healthy for anyone, and you'd likely regret it when you end with a fat adult dog. What is needed for building lean muscle mass is actually protein. The best source of protein is meat - so the wisest thing to be adding to her diet is things like meat (preferably red meat), oily fish such as sardines or mackeral (not tuna), an egg once or twice a week, and possibly a little bit of cottage cheese now and again. These are all good healthy additions that will improve her diet rather than loading it up with unneeded fat, and could be continued throughout her life (just don't forget to cut back a little on the kibble to compensate, when she's no longer lean

).