Special Characters (particularly ?) not working in custom redirect URL field

I'm attempting to redirect to a page with a question mark in the URL so that I can track submissions in Google Analytics. A form submission would ideally redirect to http://example.com/application-completed?abc123 (abc123 being a replacement token taken from a hidden field in the form, an ID if you like).

The issue I've got is that the question mark causes the URL to cut off at that point, so the above example simply redirects to http://example.com/application-completed. If I replace the ? with a /, the redirect works so I know the replacement token is correct. The reason I need to use the ? is that I have created an 'application-completed' page that uses a custom template so going to application-completed/abc123 gives me a 404.

I'm guessing it is something to do with valid_url? Any help would be much appreciated. I'm a front end dev with little knowledge of altering Drupal beyond the standard backend so please forgive me if the question seems basic.