magento-test/dev/tests/integration/testsuite/Magento/Sales/etc/extension_attributes.xml

<?xml version="1.0"?>
<!--
/**
 * Copyright © Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */
-->
<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\ShipmentInterface">
        <attribute code="test_attribute_value" type="string"/>
    </extension_attributes>
    <extension_attributes for="Magento\Sales\Api\Data\ShipmentCreationArgumentsInterface">
        <attribute code="test_attribute_value" type="string"/>
    </extension_attributes>
</config>