Shoutbox

you need to know the checkbox's handle:
var CheckBoxHwnd = PlusWnd.GetControlHandle("your_checkbox_ControlId");

// set the "Partial Check" state
Interop.Call("user32", "SendMessageW", CheckBoxHwnd, 0xF1, 0xF1, 0)


Edit: 
w.SendControlMessage(...)
woorks too, but i like API