You must specify the identifier of a current chat session, and a chat option from the list described for the routine GetChatOption.
For numeric options (such as the flags) the supplied string will be converted to a number. It is an error if the string does not consist of digits (0 to 9).
For boolean options (such as "ignore") the supplied string may be:
"y", "Y", or "1" for true
"n", "N", or "0" for false
Also if you are using Lua you can supply simply true or false.
Note that because chat sessions are transitory the option is not remembered in the world file or anywhere else for future sessions. Thus, any options must be set on a per-session basis.
See the supplied Chat.xml plugin for an example of how you can do that automatically as a new session is started.
You can use true or false for boolean options, as these are converted to 1 or 0.
Returns
eChatIDNotFound: That chat ID is not a current chat session.
eOptionOutOfRange: The numeric option is out of range.
ePluginCannotSetOption: The option is marked as non-settable (in a plugin or not)
eUnknownOption: Option name is not known.
eOK: Set the option OK.