Octopus-API/Octopus-Client works on Project Id or SlugId, However, there is no way to see these values in the web app. It would be intuitive if they are included in the project settings (albeit read-only) for usage across Octopus tools.

Usage:
https://help.octopus.com/t/can-we-see-the-project-id-in-octopus-web-app