POST method is not working in Codeigniter v4 - codeigniter

Question

I'm using form_open() for open form but form method is always showing get.
echo $this->request->getMethod() showing always get.
generated HTML is:
<form action="http://darkadmin.com/admin" method="post" accept-charset="utf-8">
<input type="hidden" name="_csrf" value="45435e3abd2d067883dacd6f62280fa7" />
<label for="fname">First name:</label>
<input type="text" id="fname" name="fname"><br><br>
<label for="lname">Last name:</label>
<input type="text" id="lname" name="lname"><br><br>
<input type="submit" value="Submit">
</form>
View:
<?php if(isset($validation)) { ?>
<?php echo $validation->listErrors(); ?>
<?php } ?>
<?php echo form_open(current_url().'admin'); ?>
<label for="fname">First name:</label>
<input type="text" id="fname" name="fname"><br><br>
<label for="lname">Last name:</label>
<input type="text" id="lname" name="lname"><br><br>
<input type="submit" value="Submit">
<?php echo form_close(); ?>
Controller:
<?php
namespace App\Controllers;
class Login extends BaseController
{
public function __construct(){
helper(['form', 'url']);
}
public function index()
{
$rules = [
'fname' => 'required',
'lname' => 'required'
];
echo $this->request->getMethod();
if($this->request->getMethod() == 'post' && $this->validate($rules)){
echo 'Success';
} else {
$this->data['validation'] = $this->validator;
}
return view($this->data['config']->theme.'login_view',$this->data);
}
}
How can I solve this problem? Please help me