di.xml

<?xml version="1.0"?>
<!--
/**
 * Copyright © Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */
-->
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    <type name="Magento\SalesRule\Helper\Coupon">
        <arguments>
            <argument name="couponParameters" xsi:type="array">
                <item name="separator" xsi:type="string">-</item>
                <item name="charset" xsi:type="array">
                    <item name="alphanum" xsi:type="string">ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789</item>
                    <item name="alpha" xsi:type="string">ABCDEFGHIJKLMNOPQRSTUVWXYZ</item>
                    <item name="num" xsi:type="string">0123456789</item>
                </item>
            </argument>
        </arguments>
    </type>
</config>