CodeIgniter 4 data submission returning page 404 - post

Question

I've just started using CodeIgniter 4 and it's very different from CI 3. When doing the form submission, either by using PHP post or ajax post request, both are returning page 404.
$('#formData').on('submit', function(e){
e.preventDefault();
var FormData = $(this).serialize();
$.ajax({
type: 'post',
dataType:'json',
url: '<?= base_url("/send-email") ?>',
data: FormData,
success: function(res){
console.log(res)
}
})
})
<form id="formData" method="post">
<div class="form-group row">
<div class="col-md-6 mb-4 mb-lg-0">
<input
type="text"
class="form-control"
placeholder="First name"
name="firstname"
/>
</div>
<div class="col-md-6">
<input
type="text"
class="form-control"
placeholder="Last name"
name="lastname"
/>
</div>
</div>
<div class="form-group row">
<div class="col-md-6 mr-auto">
<input type="submit" id="send_email" class="btn btn-block btn-primary text-white py-3 px-5" value="Send Message">
</div>
</div>
</form>
PHP Code
<?php namespace App\Controllers;
use CodeIgniter\Controller;
class DefaultController extends BaseController
{
public function send_email(){
if($this->request->isAJAX){
print_r($_POST);
}
}
}
I have form helper loaded from BaseController. I wonder if there is any other setting that need to be configured to make it work. I totally don't have any idea why.

Answer 1

Okay I have figured out the cause of the error. In order to send any request, I have to specify it in the routes of the specific method that I want to use first. In my case, I am trying to send a post request. So my routing should be like this :
$routes->post('/send-email', 'DefaultController::send_email');
Base on the doc https://codeigniter.com/user_guide/incoming/routing.html#using-http-verbs-in-routes