ASP.NET Core AJAX not working?

Hi,

I am trying to implement AJAX in ASP.NET Core so I have created a new default "ASP.NET Core Web Application (.NET Core) with "Individual User Accounts".
I added "Microsoft.jQuery.Unobtrusive.Ajax": "3.2.3" at project.json under "dependencies" section.
for "Register.cshtml" I have changed the declaration to

<form asp-controller="Account" asp-action="Register" asp-route-returnurl="@ViewData["ReturnUrl"]" data-ajax="true" data-ajax-method="POST" class="form-horizontal">

at "Register" action in AccountController I have changed it to check if the post is using AJAX.

[HttpPost]
    [AllowAnonymous]
    [ValidateAntiForgeryToken]
    public async Task<IActionResult> Register(RegisterViewModel model, string returnUrl = null)
    {
        var isAjax = HttpContext.Request.Headers["X-Requested-With"] == "XMLHttpRequest";

        ViewData["ReturnUrl"] = returnUrl;
        if (ModelState.IsValid)
        {
            var user = new ApplicationUser { UserName = model.Email, Email = model.Email };
            var result = await _userManager.CreateAsync(user, model.Password);
            if (result.Succeeded)
            {
                // For more information on how to enable account confirmation and password reset please visit http://go.microsoft.com/fwlink/?LinkID=532713
                // Send an email with this link
                //var code = await _userManager.GenerateEmailConfirmationTokenAsync(user);
                //var callbackUrl = Url.Action("ConfirmEmail", "Account", new { userId = user.Id, code = code }, protocol: HttpContext.Request.Scheme);
                //await _emailSender.SendEmailAsync(model.Email, "Confirm your account",
                //    $"Please confirm your account by clicking this link: <a href='{callbackUrl}'>link</a>");
                await _signInManager.SignInAsync(user, isPersistent: false);
                _logger.LogInformation(3, "User created a new account with password.");
                return RedirectToLocal(returnUrl);
            }
            AddErrors(result);
        }

        // If we got this far, something failed, redisplay form
        return View(model);
    }

The problem is that isAjax always returns false.

Thanks in Advanced for any help,
Asaf