func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {
  
    Purchases.logLevel = .debug
    Purchases.configure(withAPIKey: "public_sdk_key")
  
}

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
    // Override point for customization after application launch.
    
    RCPurchases.logLevel = RCLogLevelDebug;
    [RCPurchases configureWithAPIKey:@"public_sdk_key"];
    
    return YES;
}

// If you're targeting only Google Play Store
class MainApplication: Application() {
    override fun onCreate() {
        super.onCreate()
        Purchases.debugLogsEnabled = true
        Purchases.configure(PurchasesConfiguration.Builder(this, "public_google_sdk_key").build())
    }
}

// If you're building for the Amazon Appstore, you can use flavors to determine which keys to use
// In your build.gradle:
flavorDimensions "store"
productFlavors {
    amazon {
        buildConfigField "String", "STORE", "\"amazon\""
    }

    google {
        buildConfigField "String", "STORE", "\"google\""
    }       
}

///...

class MainApplication: Application() {
    override fun onCreate() {
        super.onCreate()
        Purchases.debugLogsEnabled = true
          
        if (BuildConfig.STORE.equals("amazon")) {
            Purchases.configure(AmazonConfiguration.Builder(this, "public_amazon_sdk_key").build())
        } else if (BuildConfig.STORE.equals("google")) {
            Purchases.configure(PurchasesConfiguration.Builder(this, "public_google_sdk_key").build())
        }
    }
}

// If you're targeting only Google Play Store
public class MainApplication extends Application {
    @Override
    public void onCreate() {
        super.onCreate();
        Purchases.debugLogsEnabled = true;
        Purchases.configure(new PurchasesConfiguration.Builder(this, "public_google_sdk_key").build());
    }
}

// If you're building for the Amazon Appstore, 
// click the Kotlin tab to see how to set up flavors in your build.gradle:
///...

public class MainApplication extends Application {
    @Override
    public void onCreate() {
        super.onCreate();
        Purchases.debugLogsEnabled = true;
      
        PurchasesConfiguration.Builder builder = null;
      
        if (BuildConfig.STORE.equals("amazon")) {
            builder = new AmazonConfiguration.Builder(this, "public_amazon_sdk_key");
        } else if (BuildConfig.STORE.equals("google")) {
            builder = new PurchasesConfiguration.Builder(this, "public_google_sdk_key");
        }
      
        Purchases.configure(builder.build());
    }
}

import 'dart:io' show Platform;

//...

Future<void> initPlatformState() async {
  await Purchases.setDebugLogsEnabled(true);
  
  if (Platform.isAndroid) {
    await Purchases.setup("public_google_sdk_key");
  } else if (Platform.isIOS) {
    await Purchases.setup("public_ios_sdk_key");
    
    // OR: if building for Amazon, be sure to follow the installation instructions then:
    await Purchases.setup("public_amazon_sdk_key", useAmazon: true);
  }

}

import { Platform } from 'react-native';

//...

export default class App extends React.Component {
 
  componentDidMount() {
    Purchases.setDebugLogsEnabled(true);
    
    if (Platform.OS === 'ios') {
        await Purchases.setup("public_ios_sdk_key");
    } else if (Platform.OS === 'android') {
        await Purchases.setup("public_google_sdk_key");
      
      // OR: if building for Amazon, be sure to follow the installation instructions then:
        await Purchases.setup({ apiKey: "public_amazon_sdk_key", useAmazon: true });
    }
    
  }
}

document.addEventListener("deviceready", onDeviceReady, false);

function onDeviceReady() {
    Purchases.setDebugLogsEnabled(true);
    if (window.cordova.platformId === 'ios') {
        Purchases.setup("public_ios_sdk_key");
    } else if (window.cordova.platformId === 'android') {
        Purchases.setup("public_google_sdk_key");
    }
    // OR: if building for Amazon, be sure to follow the installation instructions then:
    await Purchases.setup({ apiKey: "public_amazon_sdk_key", useAmazon: true });
}

See Unity installation instructions https://docs.revenuecat.com/docs/unity