magento-test/app/code/Magento/Newsletter/Test/Mftf/ActionGroup/SwitchToPreviewIframeActionGroup.xml

<?xml version="1.0" encoding="UTF-8"?>
<!--
 /**
  * Copyright © Magento, Inc. All rights reserved.
  * See COPYING.txt for license details.
  */
-->
<actionGroups xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
              xsi:noNamespaceSchemaLocation="urn:magento:mftf:Test/etc/actionGroupSchema.xsd">
    <actionGroup name="SwitchToPreviewIframeActionGroup">
        <executeJS function="document.getElementById('preview_iframe').sandbox.add('allow-scripts')" stepKey="addSandboxValue"/>
        <wait time="10" stepKey="waitBeforeSwitchToIframe"/>
        <switchToIFrame userInput="preview_iframe" stepKey="switchToIframe" />
        <waitForPageLoad stepKey="waitForPageLoad"/>
    </actionGroup>
</actionGroups>