Not showering during confinement is a myth in Singapore's context (also acknowledged by Eu Yan Sang https://www.euyansang.com.sg/en/beware-these-6-confinement-myths/eysfertility1.html). Keeping basic hygiene is important, especially since you will be handling a newborn in close contact. Bathing with warm water is sufficient for keeping the "wind" out of your body, if that is what your MIL is concerned about. Blow drying your hair after a shower probably helps too. Caring for a newborn can really work up a sweat, so please do keep clean and hygienic for the sake of your baby!
I have used dried shampoo before but I find it very sticky and umfortable after using. During confinement I purchase herbs from chinese medical hall to bath to expel tummy wind and keep body warm.
I'd rather just shower with warm water to maintain hygiene instead of resorting to these products. You can use herbal confinement bath water to shower with as well.
Hmmm...Maybe Google for more info regarding this product...
thanks