I created a Group Chat Service named talk (talk.xmpp.example.com) and a room named room1 (email@example.com).
What should be done or/and what plugin should be installed so that an HTTP request for access to this room can be sent to (http://xmpp.example.com/join/room1)?
How exactly did you come to this URL? I do not quite recognize it.
XMPP functionality is primarily exposed through the XMPP protocol. There is a HTTP binding to XMPP called ‘BOSH’ (or “HTTP-Bind”), but that does not expose a URL like that. Finally, there is the REST API plugin for Openfire, but that too cannot be used like this.
The closest thing that I can think of is that you install some kind of web client that you host on that URL. Openfire provides some plugins that automatically install a web client (like Converse and JSXC), but those would expose the web client on a different URL than the one that you are showing.