New feature just added into Pokémon GO for Android user, now you can redeem in-game items for free by entering promo codes in game’s Shop. The support page on official Pokémon GO website stated that this new feature allow Android players to type in promo codes for free items included Poké Balls, Lure Modules, Lucky Eggs and others.
A new 'Promo Code' field appeared briefly before vanishing, hinting that codes may soon be redeemable in-store, travelers. pic.twitter.com/sMzma26S0l
— The Silph Road (@TheSilphRoad) May 2, 2017
To enter promo code, go to Map View > Main Menu button > Shop button > type Promo code at bottom > Redeem button. Then the free in-game items will list in your inventory.
At present, there are no available codes. Niantic stated they will occasionally offers promotional codes through partnerships and special events.
Promo Codes FAQ – Pokemon Go’s new Promo Codes are just PoGo’s version of Ingress’s Passcodes. I’m an ingress player who knows a lot about Ingress Passcodes. I’ve made a little FAQ. If there’s anything I missed AMA. from TheSilphRoad
Source from Pokémon GO Support Page: Redeeming promo codes (Android only)