magento-test/app/code/Magento/Sales/Test/Mftf/ActionGroup/AdminApplyCouponToOrderActionGroup.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="AdminApplyCouponToOrderActionGroup">
        <arguments>
            <argument name="couponCode" type="string"/>
        </arguments>
        <fillField selector="{{AdminOrderFormItemsSection.couponCode}}" userInput="{{couponCode}}" stepKey="fillCouponCode"/>
        <click selector="{{AdminOrderFormItemsSection.applyCoupon}}" stepKey="applyCoupon"/>
        <waitForPageLoad stepKey="waitForApplyingCoupon"/>
        <see selector="{{AdminOrderDetailsMessagesSection.successMessage}}" userInput="The coupon code has been accepted." stepKey="seeSuccessMessage"/>
    </actionGroup>
</actionGroups>