Notifications and ViewControllers

Please copy the relevant part of code as well, don't force us to go and inspect a whole code.

AppDelegate:

Code Block //
import UIKit
import Firebase
@main
class AppDelegate: UIResponder, UIApplicationDelegate {
    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
        // Override point for customization after application launch.
        FirebaseApp.configure()
        return true
    }
    // MARK: UISceneSession Lifecycle
    func application(_ application: UIApplication, configurationForConnecting connectingSceneSession: UISceneSession, options: UIScene.ConnectionOptions) -> UISceneConfiguration {
        // Called when a new scene session is being created.
        // Use this method to select a configuration to create the new scene with.
        return UISceneConfiguration(name: "Default Configuration", sessionRole: connectingSceneSession.role)
    }
    func application(_ application: UIApplication, didDiscardSceneSessions sceneSessions: Set<UISceneSession>) {
        // Called when the user discards a scene session.
        // If any sessions were discarded while the application was not running, this will be called shortly after application:didFinishLaunchingWithOptions.
        // Use this method to release any resources that were specific to the discarded scenes, as they will not return.
    }
}

TheRiddle ViewController:

Code Block import UIKit
import SafariServices
import WebKit
import Firebase
class TheRiddle: UIViewController {
    
    @IBOutlet weak var leading_TheRiddle: NSLayoutConstraint!
    @IBOutlet weak var trailing_TheRiddle: NSLayoutConstraint!
    
    @IBOutlet weak var CurrentRiddle: UILabel!
    @IBOutlet weak var PreviousRiddle: UILabel!
    
    var menuOut = false
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        let db = Firestore.firestore()
        
        db.collection("TheRiddle").document("Riddles").getDocument { (document, error) in
            
            //check for error
            if error == nil {
                //check if document exists
                if document != nil {
                }else {
                    if let currentRiddle = document!.get("CurrentRiddle") as? String {
                        self.CurrentRiddle.text = "This Weeks Riddle: " + currentRiddle
                    }
                    if let previousRiddle = document!.get("CurrentRiddle") as? String {
                        self.CurrentRiddle.text = "Previous Riddle: " + previousRiddle
                    }
                }
            }
            
        }
        
        
    }
    
    
    @IBAction func RiddleAnswerForm(_ sender: Any) {
        let vc = SFSafariViewController(url: URL(string: "https://forms.office.com/Pages/ResponsePage.aspx?id=ytAqTDte6UK-KD5v_kOm4Y843IzqmYtFlDtLrfRYsi1UMFpUMk1GN01GS05BVFlJUElONk4yR1hKUCQlQCN0PWcu")!)
        
        present(vc, animated: true)
    }
    
    @IBAction func menuTappedTheRiddle(_ sender: Any) {
        
        if menuOut == false {
            leading_TheRiddle.constant = 150
            trailing_TheRiddle.constant = -150
            menuOut = true
        }
        else {
            leading_TheRiddle.constant = 0
            trailing_TheRiddle.constant = 0
            menuOut = false
        }
        UIView.animate(withDuration: 0.2, delay: 0.0, options: .curveEaseIn, animations: {
            self.view.layoutIfNeeded()
        }) { (animationComplete) in
            print("The animation is complete")
        }
}
}

First, I formatted your code to make it easier to grasp.
I see nowhere any reference to notification.

How do you want to open ?

• when user taps the notification ?

• other ?

What is it more specifically you don't know how to do ?

For User Notification, I advise to read first this tutorial, which explain the whole process from registering to processing notification.
https ://www.raywenderlich. com/11395893-push-notifications-tutorial-getting-started

Code Block 
import UIKit
import Firebase
@main
class AppDelegate: UIResponder, UIApplicationDelegate {
    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
        FirebaseApp.configure()
        return true
    }
    // MARK: UISceneSession Lifecycle
    func application(_ application: UIApplication, configurationForConnecting connectingSceneSession: UISceneSession, options: UIScene.ConnectionOptions) -> UISceneConfiguration {
        return UISceneConfiguration(name: "Default Configuration", sessionRole: connectingSceneSession.role)
    }
    func application(_ application: UIApplication, didDiscardSceneSessions sceneSessions: Set<UISceneSession>) {
    }
}
// TheRiddle ViewController:
import UIKit
import SafariServices
import WebKit
import Firebase
class TheRiddle: UIViewController {
    
    @IBOutlet weak var leading_TheRiddle: NSLayoutConstraint!
    @IBOutlet weak var trailing_TheRiddle: NSLayoutConstraint!
    
    @IBOutlet weak var CurrentRiddle: UILabel!
    @IBOutlet weak var PreviousRiddle: UILabel!
    
    var menuOut = false
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        let db = Firestore.firestore()
        
        db.collection("TheRiddle").document("Riddles").getDocument { (document, error) in
            
            //check for error
            if error == nil {
                //check if document exists
                if document != nil {
                } else {
                    if let currentRiddle = document!.get("CurrentRiddle") as? String {
                        self.CurrentRiddle.text = "This Weeks Riddle: " + currentRiddle
                    }
                    if let previousRiddle = document!.get("CurrentRiddle") as? String {
                        self.CurrentRiddle.text = "Previous Riddle: " + previousRiddle
                    }
                }
            }
        }
    }
    
    
    @IBAction func RiddleAnswerForm(_ sender: Any) {
        let vc = SFSafariViewController(url: URL(string: "https://forms.office.com/Pages/ResponsePage.aspx?id=ytAqTDte6UK-KD5v_kOm4Y843IzqmYtFlDtLrfRYsi1UMFpUMk1GN01GS05BVFlJUElONk4yR1hKUCQlQCN0PWcu")!)
        
        present(vc, animated: true)
    }
    
    @IBAction func menuTappedTheRiddle(_ sender: Any) {
        
        if menuOut == false {
            leading_TheRiddle.constant = 150
            trailing_TheRiddle.constant = -150
            menuOut = true
        }
        else {
            leading_TheRiddle.constant = 0
            trailing_TheRiddle.constant = 0
            menuOut = false
        }
        UIView.animate(withDuration: 0.2, delay: 0.0, options: .curveEaseIn, animations: {
            self.view.layoutIfNeeded()
        }) { (animationComplete) in
            print("The animation is complete")
        }
    }
}

Thank you so much. Will check out the tutorial.

OK. Please tell if you have any problem implementing. Otherwise, don't forget to close the thread on the correct answer.