newsupernova/vendor/magento/module-currency-symbol/view/adminhtml/templates/grid.phtml

<?php
/**
 * Copyright © Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */

// @codingStandardsIgnoreFile

?>
<?php
/**
 * @var $block \Magento\CurrencySymbol\Block\Adminhtml\System\Currencysymbol
 */
?>

<form id="currency-symbols-form" action="<?= /* @escapeNotVerified */ $block->getFormActionUrl() ?>" method="post">
    <input name="form_key" type="hidden" value="<?= /* @escapeNotVerified */ $block->getFormKey() ?>" />
    <fieldset class="admin__fieldset">
        <?php foreach ($block->getCurrencySymbolsData() as $code => $data): ?>
        <div class="admin__field _required">
            <label class="admin__field-label" for="custom_currency_symbol<?= /* @escapeNotVerified */ $code ?>">
                <span><?= /* @escapeNotVerified */ $code ?> (<?= /* @escapeNotVerified */ $data['displayName'] ?>)</span>
            </label>
            <div class="admin__field-control">
                <input id="custom_currency_symbol<?= /* @escapeNotVerified */ $code ?>"
                       class="required-entry admin__control-text <?= $data['inherited'] ? 'disabled' : '' ?>"
                       type="text"
                       value="<?= $block->escapeHtmlAttr($data['displaySymbol']) ?>"
                       name="custom_currency_symbol[<?= /* @escapeNotVerified */ $code ?>]">
                <div class="admin__field admin__field-option">
                    <input id="custom_currency_symbol_inherit<?= /* @escapeNotVerified */ $code ?>"
                           class="admin__control-checkbox" type="checkbox"
                           onclick="toggleUseDefault(<?= /* @escapeNotVerified */ '\'' . $code . '\',\'' . $block->escapeJs($data['parentSymbol']) . '\'' ?>)"
                            <?= $data['inherited'] ? ' checked="checked"' : '' ?>
                           value="1"
                           name="inherit_custom_currency_symbol[<?= /* @escapeNotVerified */ $code ?>]">
                    <label class="admin__field-label" for="custom_currency_symbol_inherit<?= /* @escapeNotVerified */ $code ?>"><span><?= /* @escapeNotVerified */ $block->getInheritText() ?></span></label>
                </div>
            </div>
        </div>
        <?php endforeach; ?>
    </fieldset>
</form>
<script>
require(['jquery', "mage/mage", 'prototype'], function(jQuery){

    jQuery('#currency-symbols-form').mage('form').mage('validation');

    function toggleUseDefault(code, value)
    {
        checkbox = jQuery('#custom_currency_symbol_inherit'+code);
        input = jQuery('#custom_currency_symbol'+code);

        if (checkbox.is(':checked')) {
            input.addClass('disabled');
            input.val(value);
            input.prop('readonly', true);
        } else {
            input.removeClass('disabled');
            input.prop('readonly', false);
        }
    }
    window.toggleUseDefault = toggleUseDefault;
});
</script>