magento-test/app/code/Magento/Newsletter/view/adminhtml/layout/preview.xml

<?xml version="1.0"?>
<!--
/**
 * Copyright © Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */
-->
<layout xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:View/Layout/etc/layout_generic.xsd">
    <container name="root">
        <block class="Magento\Framework\View\Element\Template" name="page.block" template="Magento_Newsletter::template/preview.phtml">
            <block class="Magento\Newsletter\Block\Adminhtml\Template\Preview" name="content" as="content"/>
        </block>
    </container>
</layout>