It's amazing how few people know this trick, but it's really quite easy. Instead of sharing a promotional code like "XC53GYL1", requiring the recipient to open iTunes and painstakingly type in the code, or worse yet, try to do so through the iOS App Store, send a link that opens iTunes or the App Store App automatically. Simply format your link as follows:

https://phobos.apple.com/WebObjects/MZFinance.woa/wa/freeProductCodeWizard?code=REPLACEWITHPROMOCODE

and replace "REPLACEWITHPROMOCODE" with your actual promo code.