<div id="whitepaper" class="home_whitepaper_bg w-clearfix">
  <?php $lpcnt=0; if(have_rows('whitepaper_list')): ?>
  <?php while(have_rows('whitepaper_list')): the_row(); $lpcnt++; ?>
<!-- Making rows for content, looping through and counting loops = lpcnt -->

<!--
... Other content ...
-->

  <?php
   the_sub_field('whitepaper_download_form');
  ?>
  <?php
   $pdf=get_permalink().'?download='.get_sub_field('whitepaper_pdf');
  ?>
  <a data-fancybox data-src="#whitepaper_popup_<?php echo $lpcnt; ?>" data-redirect="<?php echo $pdf; ?>" href="javascript:;" class="whitepaper_download_link w-inline-block w-clearfix">
<!-- Here's the buttons for fancybox popups. They use the loop count to build a specific url - this is info I want later! -->

<div id="whitepaper_popup_<?php echo $lpcnt; ?>" class="whitepaper_popup">
<!-- using that loop count to get a specific div id! -->
        <div class="popup_whitepaper">
            <div class="whitepaper_form">
                <!--
I could put a <?php echo do_shortcode...> here, no?
Their current solution is to grab a javascript code from an ACF field.
I have had a lot of trouble figuring out how to put the CF7 form in that field.
                -->
                <?php the_sub_field('download_form'); ?>
                <div class="w-clearfix"></div>
            </div>
            <button data-fancybox-close="" class="fancybox-close-small"></button>
            <div class="w-clearfix"></div>
        </div>
    </div>

add_action( 'wp_footer', 'mycustom_wp_footer' );

function mycustom_wp_footer() {
?>
<script type="text/javascript">
document.addEventListener( 'wpcf7mailsent', function( event ) {
if ( 'FORM ID' == event.detail.contactFormId ) {
    location = <!-- specific url derived from $lpcnt? -->;
}
}, false );
</script>
<?php
}