How to Get Value of Custom Attribute via Rest API Magento 2

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Api/etc/extension_attributes.xsd">
  <extension_attributes for="Magento\Sales\Api\Data\OrderInterface">
      <attribute code="mageplaza_helloworld_custom_attribute" type="string" />
  </extension_attributes>
</config>

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:noNamespaceSchemaLocation="urn:magento:framework:Event/etc/events.xsd">
  <event name="sales_order_load_after">
      <observer name="sales_order_load_helloworld_custom_attribute" instance="Mageplaza\HelloWorld\Observer\Sales\OrderLoadAfter" />
  </event>
</config>

<?php
namespace Mageplaza\HelloWorld\Observer\Sales;

use Magento\Framework\Event\ObserverInterface;

class OrderLoadAfter implements ObserverInterface
 
{
 
public function execute(\Magento\Framework\Event\Observer $observer)
 
{
 
    $order = $observer->getOrder();
 
    $extensionAttributes = $order->getExtensionAttributes();
 
 
 
    if ($extensionAttributes === null) {
 
        $extensionAttributes = $this->getOrderExtensionDependency();
 
    }
 
    $attr = $order->getData('helloworld_custom_attribute');
 
    $extensionAttributes->setHelloWorldCustomAttribute($attr);
 
    $order->setExtensionAttributes($extensionAttributes);
 
}
 
   
 
private function getOrderExtensionDependency()
 
{
 
    $orderExtension = \Magento\Framework\App\ObjectManager::getInstance()->get(
        '\Magento\Sales\Api\Data\OrderExtension'
    );
 
    return $orderExtension;
 
}
 
}

/**
  * @return string|null
  */
public function getHelloWorldCustomAttribute()
{
     return $this->_get('helloworld_custom_attribute');
}

/**
  * @param string $helloWorldCustomAttribute
  * @return $this
  */
public function setHelloWorldAttribute($helloWorldCustomAttribute)
{
	 $this->setData ('helloworld_custom_attribute', $helloWorldCustomAttribute);
	 return $this;
}