magento-test/app/code/Magento/Checkout/Test/Mftf/ActionGroup/ClickPlaceOrderActionGroup.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="ClickPlaceOrderActionGroup">
        <annotations>
            <description>Clicks on the 'Place Order' button. Validates that the Success Message is present and correct.</description>
        </annotations>

        <waitForElement selector="{{CheckoutPaymentSection.placeOrder}}" time="30" stepKey="waitForPlaceOrderButton"/>
        <click selector="{{CheckoutPaymentSection.placeOrder}}" stepKey="clickPlaceOrder"/>
        <waitForPageLoad stepKey="waitForCheckout"/>
        <see selector="{{CheckoutSuccessMainSection.successTitle}}" userInput="Thank you for your purchase!" stepKey="waitForLoadSuccessPage"/>
    </actionGroup>
</actionGroups>