URL for redeeming Mac App Store promo codes
The Mac App Store has two URL schemes, macappstore: and macappstores:. It looks like you need to use latter (with the trailing s) for secure links. The following link format seems to work for sending out direct links to redeem a promo code:
macappstores://buy.itunes.apple.com/WebObjects/MZFinance.woa/wa/redeemLandingPage?code=YOUR_PROMO_CODE
I've tried macappstores://... which didn't work.
but this works:
http://buy.itunes.apple.com/WebObjects/MZFinance.woa/wa/redeemLandingPage?code=YOUR_PROMO_CODE
(I tested on iOS 7 on the iPhone)
If you use itmss it works OK when opening the link in iOS and Mac/PC.
itmss://buy.itunes.apple.com/WebObjects/MZFinance.woa/wa/freeProductCodeWizard?code=THE_CODE