To enable in app
purchasing capabilities with your GameMaker: Studio title,
you should check the "Enable In App Purchase" check-box
available on this tab. If you are wanting to test the IAP
functionality on Android, iOS, WindowsPhone or Tizen, then you
should check the "Sandbox Mode" box too, which will enable
the different sandboxing methods used by the given store.
Below this is the Android specific section, where you can mark your game as using the Amazon Store if required, and you can also supply your Android Public Key for Google Play too. This is a key code supplied by Google as a protection method for your game. This licence can be found by going to your Google Play Developers Page and then selecting the option Edit profile. Once on the profile page, scroll down to the section titled "Licensing & In-app Billing" and there you should find the Public key that is needed for this functionality to be enabled.
Finally, if you are creating IAP for Tizen, you can specify here the "Item Group ID" for the game (as set up on the Tizen web dashboard).
For more detailed information on in app purchases, the available GameMaker: Studio functions and proprietary server setup, please see the section Reference - In App Purchases.
NOTE: This functionality is limited to the Professional Version of GameMaker: Studio.