Primary menu item is not highlighting when page is active even though it is linked from a url with query string to pre-populate a form field
I checked both of your links and in the <body> tag, both of them have class page-id-1508, meaning that the page id is recognised correctly either with or without the query variable. However, as @mmm suggested in the comment above, it seems that you added the menu item as Custom Link (note the menu item … Read more