Celigo doesn't assign ISA IDs, so you will need to find or define it independently. Your ISA ID can be mutually defined or can be your Dun & Bradstreet ID. If you don't know your D&B ID, you can look it up here:
https://www.dnb.com/duns-number/lookup.html
A phone number is also commonly used as an ISA ID.
For more general information on the ISA ID, see this article from True Commerce.
Let us know if you have other questions about how integrator.io handles EDI data.