If you are looking to launch Jabber into an IM session from another application, you may find the cross-launch URI capabilitiy helpful. Like the 'mailto:' URI which is registered in Windows to launch the system-default mail client, Jabber can be crosslaunched by other apps by invoking the 'tel:' (basic call) or 'xmpp:' URIs. E.g.:
This will not send the message, but it can open a chat window and pre-fill the message area. That is by design. You don't want someone planting links in their web page that allows anyone to send messages for you.
The XMPP spec for XEP 147 has additional details (not sure if Jabber supports all of the extra parameters...)