/SAY_TO [-n|-h] [-q] [-f file] jid text

Send the "text" message to the specified jid.
Please note that you'll have to use a full jid with a resource identifier if you want to specify exactly where the message should go.
You can send a message to a specific resource of the currently selected contact by using /say_to ./resourcename message.
The "-n" flag turns the message to "normal" type, "-h" to "headline".  "--" can be used to send chat messages beginning with -n or -h.
When "-q" is used, the message will be sent in the background and will not change the current active window.
A text file can be provided with the "-f" switch (in which case there's no need to pass a text argument after the jid, of course).