As you've just found out, toolstrip buttons aren't actually separate controls in the windows messaging world. This is also true of menu items and some other controls.
To cause a click using a windows message, you need to send a WM directly to the toolbar, not the button, for example TB_PRESSBUTTON (http://msdn.microsoft.com/en-us/library/windows/desktop/bb787389(v=vs.85).aspx).
You have to use the SendMessage
WinAPI function, targeted at the toolbar (you can get the hWnd as usual), with TB_PRESSBUTTON as message type, the command identifier as wParam and 1 as lParam.