You can use somthing like this. (Untested)
Code:
on ^*:text:*:#: {
 if (word iswmcs $1-) { haltdef | echo -mt This is a new text. }
 elseif (word2 iswmcs $1-) { haltdef | echo -mt This is a new text. }
}

You can look at the commands by type in mirc:

/help halting default text
/help on text