There are a number of ways, according to the Goggles manifest.
The app listens for the following intent:
Intent intent = new Intent("com.google.zxing.client.android.SCAN");
startActivity(intent);
This is the code you're looking for, which is basically the same information provided by ZXing.
As an aside, you can also use a VIEW intent with path http://www.google.com/m/products/scan
though this will offer both the browser and Goggles as handlers for the uri. This webpage actually recommends installing zxing's barcode scanner rather than Google Goggles.