Comunication between C# and javaScript in CEF

CEF_JS_CSHARP.cs

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Text.RegularExpressions;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.IO;
using System.Windows.Forms;
using CefSharp.WinForms;
using CefSharp;

// This is the code for your desktop app.
// Press Ctrl+F5 (or go to Debug > Start Without Debugging) to run your app.

namespace DesktopApp1
{
    public partial class Form1 : Form
    {
        public  ChromiumWebBrowser browser;
        public class CallbackObj
        {
            public void showMessage(string msg)
            {
                // do whatever    
                Console.WriteLine(msg);
            }
            public void getImage(string imgData)
            {
                
                using (Image image = Image.FromStream(new MemoryStream(Convert.FromBase64String(GetBase64FromJavaScriptImage(imgData)))))
                {
                    // <parameters>  : path / format 
                    image.Save("output.jpg", System.Drawing.Imaging.ImageFormat.Jpeg); 
                }
            }
          
            ///  From https://dejanstojanovic.net/aspnet/2018/january/javascript-base64-image-to-bitmap-in-c/
   
            public static String GetBase64FromJavaScriptImage(String imgData)
            {
                
                return  Regex.Match(imgData, @"data:image/(?<type>.+?),(?<data>.+)").Groups["data"].Value;
                
            }
        }

        public void InitBrowser()
        {
            CefSettings settings = new CefSettings();
            settings.CefCommandLineArgs.Add("enable-media-stream", "1");
            Cef.Initialize(settings);
            CefSharpSettings.LegacyJavascriptBindingEnabled = true;
           
            browser = new ChromiumWebBrowser("localhost:3000");

          
            browser.JavascriptObjectRepository.Register("callbackObj", new CallbackObj(), isAsync: false, options: BindingOptions.DefaultBinder);
      
            this.Controls.Add(browser);
            browser.Dock = DockStyle.Fill;
        }
        public Form1()
        {
            InitializeComponent();
            InitBrowser();
        }

        private void linkLabel1_LinkClicked(object sender, LinkLabelLinkClickedEventArgs e)
        {
            // Click on the link below to continue learning how to build a desktop app using WinForms!
            System.Diagnostics.Process.Start("http://aka.ms/dotnet-get-started-desktop");

        }

        private void button1_Click(object sender, EventArgs e)
        {
            MessageBox.Show("Thanks!");
        }
    }
}