Like the manual states: https://www.scirra.com/manual/173/iap
Each operating system has a different way of configuring in-app purchases. Please refer to vendor documentation for further help. Links are provided below.
iOS: configuring with iTunes Connect
Android: Google Play In-App Billing
Windows Store: MSDN documentation
Blackberry 10: Blackberry payment service
Amazon Appstore: Amazon documentation
Please note vendor documentation may refer to code samples. You can ignore these since Construct 2's IAP plugin handles this for you. The main parts of the documentation you need to refer to are how to register seller accounts, how to add the items available for sale to the app, and testing and publishing. If a store has a test or developer mode at all, then the Test mode property of the IAP object will enable or disable that mode for the store.
I told my dentist I had trouble with my teeth and asked her to fix it without looking in my mouth..