Razor Pages automatically generates and validates Antiforgery tokens to prevent CSRF attacks. Since you aren’t sending any token within your AJAX callback, the request fails.

To solve this problem you will have to:

1. Register the Antiforgery-Service
2. Add the token to your request
3. Add the antiforgery token to your page either by adding a <form> or by directly using the @Html.AntiForgeryToken HtmlHelper

1. Register the Antiforgery-Service in your Startup.cs

public void ConfigureServices(IServiceCollection services)
{
  services.AddRazorPages();
  services.AddAntiforgery(o => o.HeaderName = "XSRF-TOKEN");
}

2. Modify your AJAX callback

In the AJAX callback we add additional code to send the XSRF-TOKEN with our request header.

$.ajax({
    type: "POST",
    url: '/?handler=YOUR_CUSTOM_HANDLER', // Replace YOUR_CUSTOM_HANDLER with your handler.
    contentType: "application/json; charset=utf-8",

    beforeSend: function (xhr) {
      xhr.setRequestHeader("XSRF-TOKEN",
        $('input:hidden[name="__RequestVerificationToken"]').val());
    },

    dataType: "json"
}).done(function (data) {
  console.log(data.result);
})

3. Add the antiforgery token to your page

You can accomplish this by adding a <form>:

<form method="post">
    <input type="button" value="Ajax test" class="btn btn-default" onclick="ajaxTest();" />
</form>

or by using the @Html.AntiForgeryToken:

@Html.AntiForgeryToken()
<input type="button" value="Ajax test" class="btn btn-default" onclick="ajaxTest();" />

In both cases Razor Pages will automatically add a hidden input field which contains the antiforgery token once the page is loaded:

<input name="__RequestVerificationToken" type="hidden" value="THE_TOKEN_VALUE" />