Customising a 404 Web Page

Hi.  Is there any way that we can get a FortiWeb to send a customised 404 page if the backend server sends a 404 error?  Just want to see if we can control this at the FortiWeb end or whether we have to set it up at the server end as I don't want the possibility that the backend server can send info about what it is running as part of the webpage.

We have customised the 500 and 503 pages but there is not an option for the 404.  Any ideas please?