You would need to run another JSP server (Apache Tomcat, Resin, or another instance of Jetty, etc) that is setup to listen on a different port (443 in your case) and have the webchat.war file in the webapps directory of that server and remove it from the Openfire plugins directory. (Tomcat and Openfire can still run on the same computer).
Tomcat for Windows can be installed as a stand alone service on the same server as IIS or use IIS to host the site and have it proxy jsp requests. (IIS doesn’t do web apps natively so you need something like Tomcat to host it and IIS can just proxy).
Essentially, many groups have a webserver that hosts external facing sites and other servers to do things like mail. In a case like this, the webchat.war web app was made to deployed on a web server (IIS + Tomcat or something like it) - call it ServerA - and there would be an OpenFire server - call it ServerB.
The Fastpath webchat.war webapp on ServerA will be configured (there’s a setup-index.jsp or something like that, or alternatively, you can edit the xml file) to talk to ServerB on the default port of 5222. It will act as a client similar to Spark, Psi, Pidgin, etc.
Now… you can also have ServerA = ServerB. IIS/Tomcat can host and run the webchat.war on some port http://server:4321/webchat and it will talk to the OpenFire service using 5222 internally. Think of it has if you installed Spark on the same machine as OpenFire.