Take it out of the </body> tag and instead add an Embed element to the form. Then add the following inside the Embed element, replacing <add cms Slug field here> with the dynamic cms field you selected:
<script>
var Webflow = Webflow || [];
Webflow.push(function() {
$('#slug').val('<add cms Slug field here>');
});
</script>
Did that fix it?
P.S.
If not, please double check that ID for your field, in your Webflow Designer, has the same spelling and case:
