What is full syntax for Delulge add attachment to an invoice?

What is full syntax for Delulge add attachment to an invoice?

Here is yet another example of unfinished documentation:
Invoices | Zoho Invoice | API Documentation

How is the "attachment" parameter specified? 
Is it a resource id?
Is it a fully-qualified url, such as a permalink or external share link?
  1. https://invoice.zoho.com/api/v3/invoices/{invoice_id}/attachment