Client ID for Google Apps Script

This won't exist automatically for scripts, and scripts don't exist in the Developer console projects, though if you go to Tools > Script Editor > Resources > Advanced Google Services the link there will lead to a project page for the script, with a link to Credentials where you can create a client ID.

The client ID is a web address string, the app ID is the first portion of the client ID (separated by a hyphen).

The example given on the Drive REST API docs for the Open command are

clientId: 1234567890-abcdefghijklmnopqrstuvwxyz.apps.googleusercontent.com
appId:    1234567890