You just ask for promo codes on itunes connect and click which app you want to get promo codes then download the promo codes and they will be on your computer. Then send these promo codes to any person with an iOS device of your choice and they just redeem it like a gift card. Also the new system allows all appstores worldwide to use promo code
In iTunes on the store front there is a link on the main page that says "Redeem", click it, you enter the promo code in there, hit OK and the rest is self explanatory. You will not be charged anything since it is a promotional code for the App.
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Cheers, Weswog