If you're browsing this answer that means you're creating an Rechatbox integration for a Mailjet Contact List.
Contact list IDs are very easy to find, just follow these simple steps below:
Step 1
Log in to your Mailjet account (https://app.mailjet.com/signin)
Step 2
Browse your contact lists page inside the Mailjet dashboard (https://app.mailjet.com/contacts)
Step 3
In side the contact lists page, for every list you will have the list ID, as seen and highlighted in the picture below.
An example List ID looks like this: 208692
That's it!