C# to retrieve list items from sharePoint online

I have console application running on my local machine, got error "System.Net.WebException: 'The remote server returned an error:(403) Forbidden.' " when executed below code Below code is working fine for SharePoint server sites but not for SharePoint online sites.

 var webUrl = new Uri("https://SharePointOnline123.com");
            using (ClientContext ctx = new ClientContext(webUrl))
            {              
                Web webObj = ctx.Web;
                List listObj = webObj.Lists.GetByTitle("AbcList");
                CamlQuery cqObj = CamlQuery.CreateAllItemsQuery(100);
                ListItemCollection colObj = listObj.GetItems(cqObj);

                ctx.Load(colObj);
                ctx.ExecuteQuery();

                if (colObj.Count != 0)
                {
                    foreach (ListItem item in colObj)
                    {
                        Console.Write($"ID: {item.Id}");
                        //Console.Write($"Title: {item.Client_Title}");
                    }
                }
             };