The code does not seem to be executed, I can not see it on the site source. I assume that custom code plugin may require a different syntax or some sort of a different configuration.
If you pleace it to the functions.php file, it will work.
If the custom code plugin allows putting custom javascript code, then you can paste only the javascript section of it to the snippet:
document.querySelectorAll('.asp_select2-search__field').forEach(function(el) {
el.disabled = true;
});Just make sure it is printed to the site footer as the last script tag.