Gets the host's IDispatch interface.
HRESULT GetExternal( IDispatch **ppDispatch );
- [out] A pointer to a variable that receives an IDispatch interface pointer for the host application.
Returns S_OK if successful, or an error value otherwise.
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.