$user_arr = $this->request->data;
$this->User->save($user_arr)

$username = $user_arr['User']['username'];
$email = $user_arr['User']['email'];
$password = $user_arr['User']['password'];
$codeLink = uniqid();

$user_arr = array('User'=>array('username'=>$username, 'email'=>$email, 'password'=>$password, '$code'=>$codeLink));

$this->request->data['User']['codeLink'] = unqueid(); // <-- this line
$this->User->create($this->request->data);
if ($this->User->save()) {
} else {
}

class User extends AppModel {

    public function register($data) {
        $data[$this->alias]['codeLink'] = uniqueid();
        $this->create($data);
        return $this->save();
    }

}

// your controller

if ($this->User->register($this->request->data) {}...