Are you certain that it's environmental allergies that's causing your dog to itch? If it's a food sensitivity or allergy that's causing it, or a flea allergy, then the shampoo is unlikely to help. Even it it's environmental the shampoo may not be enough.
My boy has environmental allergies, and my vet told me to use HyLyt Shampoo. It's basically a soap free shampoo that can be used every day. It didn't do much and I now have my boy on Apoquel during the warm months when his allergies flare up.
the vet wanted to put him on Apoquel - but my old Cairn terrier was on that and unknown to us he had cancer - and it made the cancer worse -- now I am scared of using it - it is autumn here in Australia now - he should be improving - he is on a grain free diet and Zyrtec