C# WebBrowser -> WebView2 solution

What platform are you building for?  .Net Core, WPF or WinForms

Mine is WPF based (long story), but I didn't use CoreWebView2InitializationCompleted.

namespace DigiwiseDOM
{
    using Microsoft.Web.WebView2.Core;
    using System;
    using System.Threading.Tasks;
    using System.Web;

    /// <summary>
    /// Interaction logic for CloudLogin.xaml
    /// </summary>
    public partial class CloudLogin : System.Windows.Window
    {
        public CloudLogin(string url)
        {
            this.InitializeComponent();

            Browser.NavigationCompleted += WebBNavigationCompleted;
            _ = InitializeBrowser(url);
        }

        public string GetOAuthCode()
        {
            this.ShowDialog();

            // The dialog will close once the user has granted access to MYOB and the OAuth code is in the URL
            // Extract the oAuth code and return it
            var uri = Browser.Source;
            var queryParams = HttpUtility.ParseQueryString(uri.Query);
            var code = queryParams["code"];

            Browser.Dispose();  // Prevented an exception that occcurred on exiting the application if we remove this line

            return code;
        }

        private async Task InitializeBrowser(string url)
        {
            var userDataFolder = Environment.GetFolderPath(Environment.SpecialFolder.ApplicationData) + "\\DigiwiseQuickExtract";
            var env = await CoreWebView2Environment.CreateAsync(null, userDataFolder);
            await Browser.EnsureCoreWebView2Async(env);
            Browser.Source = new UriBuilder(url).Uri;
        }

        private void WebBNavigationCompleted(object sender, EventArgs e)
        {
            var webB = (Microsoft.Web.WebView2.Wpf.WebView2)sender;

            // Check if OAuth code has been returned and close dialog if it has
            var uri = webB.Source.ToString();
            if (uri.Contains("code="))
            {
                this.Close();
            }
        }
    }
}

CoreWebView2InitializationCompleted is a public event of WebView2 in the Microsoft.Web.WebView2.WinForms namespace. If your project is WinForms, you need to ascertain why your WebView2 class is not referencing Microsoft.Web.WebView2.WinForms. 

Right-click on 'WebView2' on this line and click 'Go to definition'

            var webView2 = new WebView2

You should end up looking at this file (scroll to the top.) Presumably you are not. You need to work out why. Recheck your references.