Apple's App-Specific Shared Secret is a unique key to receive receipts for your app's auto-renewable subscriptions. This key allows you to verify these in-app purchases.
1. Sign into your Apple Developer Account and choose App Store Connect in the left side panel.
2. Click Go to App Store Connect on the right side.
3. On the App Store Connect page click My Apps. If you don't have an app, click + to create a new app.
4. Select your app and navigate to the Features tab.
5. Select In-App Purchases in the left side panel.
6. Click App-Specific Shared Secret and click Generate in the pop-up window if you don't already have one.
7. Copy your Shared Secret to be used in your app. In your app dashboard under Settings > Basic, paste it into the Shared Secret text box in the iOS card.