magento-test/app/code/Magento/Sales/Test/Mftf/Section/AdminOrderFormDiscountSection.xml

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

<sections xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
          xsi:noNamespaceSchemaLocation="urn:magento:mftf:Page/etc/SectionObject.xsd">
    <section name="AdminOrderFormDiscountSection">
        <element name="fieldCouponCode" type="input" selector="//input[@name='coupon_code']"/>
        <element name="applyCouponCode" type="button" selector="//input[@name='coupon_code']/following-sibling::button"/>
    </section>
</sections>