To assist you to find the correct LSPN there's a link on that page to search the Medicare website. Once you've entered the LSPN into the Hospitals database it will be retained for future use. It will be transmitted automatically with electronic claims when the item number requires it.
For printed invoices, you will need to add a new line to the list of items on the invoice and enter the LSPN manually.
NOTE: you should also set the referral type on the invoice to Not Required before submitting it.