"isAJAX()" method always return false;

I tried to create an API with a script like the one below. when I run it with $.ajax , the application always returns the "false" value in the isAJAX method. Whats wrong?
Thanks before.

PHP Code:

<?php 
namespace App\Controllers\Api;

use function time;
use CodeIgniter\API\ResponseTrait;
use CodeIgniter\Controller;
use CodeIgniter\HTTP\ResponseInterface;

final class Test extends Controller 
{
    use ResponseTrait;

    public function index(): ResponseInterface
    {
        return $this->respond([
            'ack' => time(),
            'req' => $this->request->getPost('foo'),
            'head' => $this->request->getServer('HTTP_BEARER'),
            'ajax' => $this->request->isAJAX(),
            ]);
    }
}