How to add custom button in toolbar

@{
    string[] items = new string[] { "NewFolder", "Upload", "Delete", "Download", "Rename", "SortBy", "Refresh", "Selection", "View", "Details", "Custom" };
}
<div class="control-section">
    <div class="sample-container">
        <!--  Filemanager element declaration -->
        @Html.EJS().FileManager("file").AjaxSettings(new Syncfusion.EJ2.FileManager.FileManagerAjaxSettings()
        {
            Url = "/Home/FileOperations",
            GetImageUrl = "/Home/GetImage",
            UploadUrl = "/Home/Upload",
            DownloadUrl = "/Home/Download"
        }).ToolbarSettings(new Syncfusion.EJ2.FileManager.FileManagerToolbarSettings()
        {
            Items = items
        }).ToolbarClick("toolbarClick").ToolbarCreate("onCreate").Render()
        <!-- end of filemanager element -->
    </div>
</div>
<script>
    // event for custom toolbar item
    function toolbarClick(args) {
        if (args.item.text === 'Custom') {
            alert('You have clicked custom toolbar item')
        }
    }
    function onCreate(args) {
        for (var i = 0; i < args.items.length; i++) {
            if (args.items[i].id === this.element.id + '_tb_custom') {
                args.items[i].prefixIcon = 'e-icons e-fe-tick';
            }
        }
    }
</script>
<style>
    .e-fe-tick::before {
        content: '\e614';
    }
</style>

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using Microsoft.AspNetCore.Mvc;

namespace WebApplication.Controllers
{
    public class HomeController : Controller
    {
        public ActionResult Index()
        {
            return View();
        }
    }
}