<p id="wpdef-2fa-email" class="<?php echo esc_attr( $class ); ?>">
	<input type="text" class="regular-text" name="def_2fa_backup_email" value="<?php echo esc_attr( $backup_email ); ?>"/>
</p>
<script type="text/javascript">
    jQuery(function ($) {
        $('body').on('click', '#field-fallback-email', function(){
            $( '#wpdef-2fa-email' ).toggle();
        });
    })
</script>