magento-test/app/code/Magento/Customer/view/frontend/templates/additionalinfocustomer.phtml

<?php
/**
 * Copyright © Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */
?>
<?php
/**
 * Customer additional info for register and login forms
 *
 */
?>
<?php if ($this->helper(\Magento\Checkout\Helper\Data::class)->isContextCheckout()) : ?>
    <input name="context" type="hidden" value="checkout" />
<?php endif; ?>