Wikipedia:Scripts/Script for XChat
To install it, copy the text below and save it into a file called wmlinksubber.py in your xchat directory and then restart XChat, or load it via the menu Window -> Plugins and Scripts....
On Unix-based systems, your XChat directory is most likely at ~/.xchat or ~/.xchat2.
On Windows systems before Windows Vista, your XChat directory is at C:\Documents and Settings\username\Application Data\X-Chat 2\. On systems running Windows Vista or later, try C:\Users\username\AppData\Roaming\XChat\addons.
If you don't want it to replace text which you type, then place a # at the beginning of the line EVENTS.append(("Your Message", 1)), and reload the plugin with the Window -> Plugins and Scripts... menu, if it is already loaded.
The script code is available at a page in its author's userspace.
If you receive an error to the effect of "so and so is not an ASCII character", add the line "# coding=utf-8" to the head of the script.