Xcode – How to call Swift from an Objective C project App Delegate?

This code is from a Swift project App delegate. It is used to help configure Stripe with a publishable key.

//Appdelegate.swift
func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: 
[UIApplicationLaunchOptionsKey: Any]?) -> Bool 
{
//The code helps configure Stripe with a publishable key.
STPPaymentConfiguration.shared().publishableKey = Constants.publishableKey
...
}

Two errors are displayed when building the app after adding the Swift line to the Objective C App Delegate

//AppDelegate.h
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary*)launchOptions
{
STPPaymentConfiguration.shared().publishableKey = Constants.publishableKey

Property 'shared' not found on object of type 'STPPaymentConfiguration'
Use of undeclared identifier 'Constants'

This was a similar error in compiling before @objc was added to the demo Swift function, MockApiClient. Should it be added elsewhere? I’ve tried adding @objc to the enum as mentioned in the answer here to no avail yet.

//Constants.swift 
//This is the file the original Swift app delegate accesses
import Foundation

  enum Constants {
  static let publishableKey = "pk_live_..."
  static let baseURLString = "http://54.33.123.227:1234"
  static let defaultCurrency = "usd"
  static let defaultDescription = "Receipt" //change to describe actual app & charge
  }

Steps taken:

1. Opened the Objective C project and created a bridging header

2. Created a demo class in Swift while still in the Obj C project to make sure it can be used, in this case to print from an Objective C file when the view is loaded. Specifically derived from an NSObject. Adding the override to the initializer and using the @objc prefix.

   //  MockApiClient.swift
   import Foundation
   class MockApiClient: NSObject
   {
   override init()
   {
   print("Initializer called in Mock API client")
   }
   @objc func executeRequest()
   {
   print("The execute request has been called in the Mock API Client")
   }
   }
   
   //ViewController.h
   //Prints the Swift request written in the MockApiClient the the view loads
   
   @implementation ViewController
   - (void)viewDidLoad
   {
   [super viewDidLoad];
   // Do any additional setup after loading the view, typically from a nib.
   MockApiClient *client = [MockApiClient new];
   [client executeRequest];
   }
   

3. Copied the #import "ViewController.h" import to the automatically generated project-Bridging-Header.h file to expose the Objective C in it to swift

4. Added the necessary Swift files to the Objective C project so that the Constants.publishablekey data from Constants.swift can be found

How can this Swift App delegate code be added to the App delegate of an Objective C project?

Edit: error when adding @objc to the enum declaration in Constants.swift