To get a listing_id you will need to have products listed on Etsy.
Note: You do not need listing_ids when creating new listings.
Once your listings are on Etsy follow these steps:
Go to the Shop page
Select the export settings you want
Click "Start Export"
Download the export
Open the download
Find the target listings and their listing_ids
Transfer them into your uploading document
With these in place, your listings will always be linked to these listing_ids.