When trying to add a new station to your organisation, you will be asked to provide its Chargebox ID.
The Chargebox ID of your charging device is basically its unique name. It does not have a predefined format and is often defined by the manufacturer, either according to their own specifications or the ones of their customer.
If you have just purchased a charging device, you should be able to find it from the box it came in. It can be sometimes also called Identity or IMEI number.
If you are dealing with a device you can't easily find this information from just by looking at it, you will find that information when connecting yourself to the station's software, usually in the General Settings.