Mock set up dispatch script when search criteria is a Form parameter

I have a mock service "GET /item/{id}" (ex: "/item/12345" to get item with id = 12345).  I need to send response 200 or 404 depending on the id on the request.  Using dispatch parameter, ID can't be identified by query nor path.  Using dispatch script, mockRequest methods to get request data are returning NULL value.  

Please help.