400 Bad Request when sending XHR from React.js to admin-ajax.php

I'm trying to send ajax requests from a React.js app to wordpress's admin-ajax.php. Here is a mini test plugin I wrote to test this:

// plugins folder  xhrLink/xhrLink.php
?php

/*
Plugin Name: Xhr Link
*/

function basicPage() {
  echo "h1Admin Page/h1";
}

function setupParams() {
  add_options_page('XHR Link', 'XHR Link', 'manage_options', 'xhr-link',  'basicPage');
}
add_action('admin_menu', 'setupParams');


function handleXhrLink() {
  wp_send_json('this response came from the back end');
}
add_action('wp_ajax_xhrLink', 'handleXhrLink');


// theme folder  functions.php
function add_cors_http_header() {
  header("Access-Control-Allow-Origin: *");
}
add_action('init','add_cors_http_header');

In the React.js app I have the following function set to fire on the onClick attribute of a button:

const sendXhrLinkRequest = () = {
      let form_data = new FormData;
      form_data.append('action', 'xhrLink');

      axios
        .post('http://localhost/wptest2/wp-admin/admin-ajax.php', form_data)
        .then(response = {
          console.log(response, `=====sendXhrLinkRequest response=====`);
      });

  };

From WordPress AJAX with Axios:

FormData interface provides a way to easily construct a set of key/value pairs representing form fields and their values, which can then be easily sent using the XMLHttpRequest.send() method. It uses the same format a form would use if the encoding type were set to "multipart/form-data".

After clicking the button the Chrome console shows this:

VM596:1 POST http://localhost/wptest2/wp-admin/admin-ajax.php 400 (Bad Request)
(anonymous) @ VM596:1

VM596:1 XHR failed loading: POST "http://localhost/wptest2/wp-admin/admin-ajax.php".

createError.js:16 Uncaught (in promise) Error: Request failed with status code 400
    at createError (createError.js:16)
    at settle (settle.js:17)
    at XMLHttpRequest.handleLoad (xhr.js:59)

Does anyone know what causes these errors and how to handle them?

Also tried:

• Submitting the form data as a JSON object. Same console entries resulted.