While SOAP and REST share similarities over the HTTP protocol. In this article, we will discuss Web API REST and Web API SOAP.

Step 1: You need to create a file in your custom module app/code/{vendor}/{module}/etc/extension_attributes.xml

<?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="seller" type="string" />
</extension_attributes>
</config>

Step 2: Create observe.In this step, you need to create observer for event ‘sales_order_load_after’ through the file {vendor}/{module}/etc/webapi_rest/events.xml

<?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_order_attribute" instance="{Vendor}\{Module}\Observer\Sales\OrderLoadAfter" />
</event>
</config>

Step 3: Create a file to handle this event.Create a file {vendor}/{module}/Observer/Sales/OrderLoadAfter.php to handle the event ‘sales_order_load_after’ which is declared above

<?php
namespace {Vendor}\{Module}\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();
    }
    
    $seller='test';//add your custom logic for seller.
    $extensionAttributes->setSeller($seller);
    $order->setExtensionAttributes($extensionAttributes);
}
private function getOrderExtensionDependency()
{
    $orderExtension = \Magento\Framework\App\ObjectManager::getInstance()->get(
        '\Magento\Sales\Api\Data\OrderExtension'
    );
    return $orderExtension;
}
}