FortiWeb - replicating F5 iRule for 404 error messaging

Hi.  I am trying to find out how I can replicate an F5 irule configuration on a FortiWeb so if the HTTP Response from the backend server shows a 404 error, I can redirect the front-end user to a different URL.  On the F5 I can do this via an iRule so I have been looking at HTTP Content Routing and cannot figure out how to replicate this on a FortiWeb.

Can anyone help please?