Gets the host's IDispatch interface.
HRESULT retVal = object.GetExternal(ppDispatch);
A pointer to a variable that receives an IDispatch interface pointer for the host application.
If this method succeeds, it returns S_OK. Otherwise, it returns an HRESULT error code.
If the host exposes an automation interface, it can provide a reference to MSHTML through ppDispatch.
If the method implementation does not supply an IDispatch, set ppDispatch to NULL, even if the method fails or returns S_FALSE.