magento-test/app/code/Magento/Customer/Test/Mftf/Metadata/CustomerNestedExtensionAttributeMeta.xml

<?xml version="1.0" encoding="UTF-8"?>
<!--
 /**
  * Copyright © Magento, Inc. All rights reserved.
  * See COPYING.txt for license details.
  */
-->

<operations xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xsi:noNamespaceSchemaLocation="urn:magento:mftf:DataGenerator/etc/dataOperation.xsd">
    <operation name="CreateNestedExtensionAttribute" dataType="customer_nested_extension_attribute" type="create">
        <field key="id">integer</field>
        <field key="customer_id">integer</field>
        <field key="value">string</field>
    </operation>
    <operation name="UpdateNestedExtensionAttribute" dataType="customer_nested_extension_attribute" type="update">
        <field key="id">integer</field>
        <field key="customer_id">integer</field>
        <field key="value">string</field>
    </operation>
</operations>