swift set initial view controller

// For Swift 4.2 and 5 code:

var window: UIWindow?

func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool { 
     self.window = UIWindow(frame: UIScreen.main.bounds)

     let storyboard = UIStoryboard(name: "Main", bundle: nil)

     let initialViewController = storyboard.instantiateViewController(withIdentifier: "dashboardVC")

     self.window?.rootViewController = initialViewController
     self.window?.makeKeyAndVisible()
}

// For Xcode 11+ and for Swift 5+ :

class SceneDelegate: UIResponder, UIWindowSceneDelegate {

     var window: UIWindow?

     func scene(_ scene: UIScene, willConnectTo session: UISceneSession, options connectionOptions: UIScene.ConnectionOptions) {
         if let windowScene = scene as? UIWindowScene {
             let window = UIWindow(windowScene: windowScene)

              window.rootViewController = // Your RootViewController in here

              self.window = window
              window.makeKeyAndVisible()
         }
    }
}