1. Create feed as shopify store in sub store. 

2. Go to main store's shopify admin find the settings below there and click on it.

3. Once click the settings, you can find the location over there. 

4. Click on the location that you want to select.

5. You can view the location_id over the url of website above. 

6. You can copy the location_id from there and paste it in the query param field.