How can I supply an AntiForgeryToken when posting JSON data using $.ajax?

You don't need the ValidationHttpRequestWrapper solution since MVC 4. According to this link.

1. Put the token in the headers.
2. Create a filter.
3. Put the attribute on your method.

Here is my solution:

var token = $('input[name="__RequestVerificationToken"]').val();
var headers = {};
headers['__RequestVerificationToken'] = token;
$.ajax({
    type: 'POST',
    url: '/MyTestMethod',
    contentType: 'application/json; charset=utf-8',
    headers: headers,
    data: JSON.stringify({
        Test: 'test'
    }),
    dataType: "json",
    success: function () {},
    error: function (xhr) {}
});


[AttributeUsage(AttributeTargets.Method | AttributeTargets.Class, AllowMultiple = false, Inherited = true)]
public class ValidateJsonAntiForgeryTokenAttribute : FilterAttribute, IAuthorizationFilter
{
    public void OnAuthorization(AuthorizationContext filterContext)
    {
        if (filterContext == null)
        {
            throw new ArgumentNullException("filterContext");
        }

        var httpContext = filterContext.HttpContext;
        var cookie = httpContext.Request.Cookies[AntiForgeryConfig.CookieName];
        AntiForgery.Validate(cookie != null ? cookie.Value : null, httpContext.Request.Headers["__RequestVerificationToken"]);
    }
}


[HttpPost]
[AllowAnonymous]
[ValidateJsonAntiForgeryToken]
public async Task<JsonResult> MyTestMethod(string Test)
{
    return Json(true);
}

Solution : 

What is wrong is that the controller action that is supposed to handle this request and which is marked with the [ValidateAntiForgeryToken] expects a parameter called __RequestVerificationToken to be POSTed along with the request.

There's no such parameter POSTed as you are using JSON.stringify(data) which converts your form to its JSON representation and so the exception is thrown.

So I can see two possible solutions here:

Number 1: Use x-www-form-urlencoded instead of JSON for sending your request parameters:

data["__RequestVerificationToken"] = $('[name=__RequestVerificationToken]').val();
data["fiscalyear"] = fiscalyear;
// ... other data if necessary

$.ajax({
    url: url,
    type: 'POST',
    context: document.body,
    data: data,
    success: function() { refresh(); }
});

Number 2: Separate the request into two parameters:

data["fiscalyear"] = fiscalyear;
// ... other data if necessary
var token = $('[name=__RequestVerificationToken]').val();

$.ajax({
    url: url,
    type: 'POST',
    context: document.body,
    data: { __RequestVerificationToken: token, jsonRequest: JSON.stringify(data) },
    success: function() { refresh(); }
});

So in all cases you need to POST the __RequestVerificationToken value.

source : https://stackoverflow.com/questions/2906754/how-can-i-supply-an-antiforgerytoken-when-posting-json-data-using-ajax