I would like to know how to make an iPad app for free for users buying a Mac app, via the appstore.
My goal is to do a promotion like "Buy Mac app and get an iPad app for free!"
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I guess you'll be limited to the 50 (?) promo code you can generate per each version of your iPad app. Sorry.
I'll suggest using a database on your website. When you buy the MAS version of the app you need to register it, this creates an entry in your DB that sets up the user has having purchased the app.
Now when they download the iDevice version of your app they're required to enter credentials that can be used to verify them against your web database. If they match up with what they entered when they set up their web account using the MAS-purchased app you let them use the iDevice app. If not, show them the screen that tells them how to buy it from the MAS first.
This a slight twist on how subscription-based apps like LastPass operate their iDevice offerings. As long as you're a LastPass premium subscriber, the iDevice apps (which are free) work. Stop subscribing and the iDevice apps stop working as they're a "premium user" feature.
The only way is to make your iPad app a Free app.
Redeem codes are limited to 50 per version, and your agreement with Apple may restrict their use.
You can buy and gift a Paid app to someone, even your own app. In the case of your own app, you will eventually get about 70% of the price back from Apple.
There is no reliable way for a developer to identify anyone who gets an App store app in any way, either by purchase, redemption code, or gift.