AppDelegate.swift

import UIKit
import Stripe

@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {

    var window: UIWindow?

    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {
        // Override point for customization after application launch.
        
        Stripe.setDefaultPublishableKey("YOUR PUBLISHABLE KEY.")
        
        return true
    }
}