if you're talking about the "SendMessage" Windows API,
code:
Interop.Call("user32","SendMessageW", hwnd, msg, wparam, lparam);
i don't use SendMessage much so this code might be wrong
otherwise, if you're talking about sending a message to a conversation window, you can look it up in the scripting documentation for "ChatWnd::SendMessage"
edit:
after looking at your code, it looks like you want to display a message box.
you can use this
code:
Interop.Call("user32","MessageBoxW",0,"title or message","title or message 2",64);
/*
32 = Error
64 = Information
all i can think of atm.
*/