How to fetch ckWebAuthToken?

I am trying to make CloudKit Web service work by following the Apple Documentation at https://developer.apple.com/library/archive/documentation/DataManagement/Conceptual/CloudKitWebServicesReference/SettingUpWebServices.html

I am able to get the redirect URL from

curl --location 'https://api.apple-cloudkit.com/database/1/iCloud.Wasiq.Migraine/development/private/users/current?ckAPIToken=b440efde65bb8c1f45464598470f4cc73977f2c10d6c40587588ec9f1c1c56f0'

Now, with the redirect URL, if I login with Apple ID, I am expecting ckWebAuthToken

Documentations says If you did not specify a custom URL when creating the API token, register an event listener with the window object to be notified when the user signs in.

window.addEventListener('message', function(e) {
console.log(e.data.ckWebAuthToken);})

Here's a complete SwiftUI working project to reproduce the issue

import SwiftUI
import UIKit
import WebKit


struct ContentView: View {
@State private var loadWeb = false
@State private var url = ""

var body: some View {
    VStack {
        Image(systemName: "globe")
            .imageScale(.large)
            .foregroundColor(.accentColor)
        Text("Hello, world!")
            .onTapGesture {
                var request = URLRequest(url: URL(string: "https://api.apple-cloudkit.com/database/1/iCloud.Wasiq.Migraine/development/private/users/current?ckAPIToken=b440efde65bb8c1f45464598470f4cc73977f2c10d6c40587588ec9f1c1c56f0")!,timeoutInterval: Double.infinity)
                request.httpMethod = "GET"
                
                let task = URLSession.shared.dataTask(with: request) { data, response, error in
                    guard let data = data else {
                        print(String(describing: error))
                        
                        return
                    }
                    do {
                        let res = try JSONDecoder().decode(Root.self, from: data)
                        print("Getting URL")
                        self.url = res.redirectURL
                        self.loadWeb = true
                        print(res.redirectURL)
                    } catch let error {
                        print(error)
                    }
                }
                
                task.resume()
                
            }
        if(!loadWeb){
            Spacer()
        }
        else{
            WebView(url: $url)
        }
    }
    
    .padding()
}
}

struct Root: Codable {
let redirectURL: String}




struct WebView: UIViewRepresentable {
@Binding var url: String


func makeCoordinator() -> WebView.Coordinator {
    Coordinator(self)
}

func makeUIView(context: Context) -> WKWebView {
    let config = WKWebViewConfiguration()
    let source = "window.addEventListener('message', function(e){ window.webkit.messageHandlers.logging.postMessage(e.data); })"
    let script = WKUserScript(source: source, injectionTime: .atDocumentStart, forMainFrameOnly: false)
    config.userContentController.addUserScript(script)
    config.userContentController.add(context.coordinator, name: "logging")
    let view = WKWebView(frame: UIScreen.main.bounds, configuration: config)
    view.navigationDelegate = context.coordinator
    view.load(URLRequest(url: URL(string: url)!))
    
    return view
}

func updateUIView(_ uiView: WKWebView, context: Context) {
    // you can access environment via context.environment here
    // Note that this method will be called A LOT
}



class Coordinator: NSObject, WKNavigationDelegate, WKScriptMessageHandler {
    
    func userContentController(_ userContentController: WKUserContentController, didReceive message: WKScriptMessage)
    {
        print("message: \(message.body)")
        // and whatever other actions you want to take
    }
    
    let parent: WebView
    
    init(_ parent: WebView) {
        self.parent = parent
    }
    

    

}}

When I tap on "Hello, World", the WebView gets loaded and asks the user to Sign in with Apple ID. After logging in, I do not see the ckWebAuthToken getting printed

Instead I see the following output

message: SOAuthorizationDidStart
message: {
authorization =     {
    "grant_code" = "ce619474218c0485182af8cf019684d2e.0.sruq.i5r0nMCZIKVEBma6p6FE5Q";
    "id_token" = "";
    state = "auth-sznjvrn0-6svd-pw2c-lwfy-47rd7n64";
};
event = AppSSOTakeoverDidComplete;
user =     {
};}
message: {
event = WidgetBridgeComplete;
state = "auth-sznjvrn0-6svd-pw2c-lwfy-47rd7n64";}