First, you'll need to make sure your store is subscribed to Selling Manager or Selling Manager Pro. Check if you're subscribed or learn more about these tools. Another thing to check for is to make sure you've completed your eBay Seller Hub verification.
Once you've done that, you should see a Custom label (SKU) field on each of your listings. You won't need to make the field yourself.
Trunk will use this field to link your listings by SKU.