hello Laika,
I also found out that you can do something like this that does the same thing…
add_action('acf/input/admin_footer','admin_footer');
function admin_footer(){
?>
<script>
acf.add_filter(
'prepare_for_ajax',
function(a) {
if (a.action === 'acf/fields/post_object/query') {
a.user_id = <?php echo isset($_GET['user_id'])?$_GET['user_id']:''; ?>;
}
return a;
}
);
</script>
<?php
}