Apprentices don't count as a seat
Upload the apprenticeship contract — and your apprentice's seat is free for as long as the apprenticeship runs.
A seat in Kundenbox is a login, and each one costs CHF 100 a month. Apprentices are the exception: once their apprenticeship contract is on file on their employment, they no longer count among the paid seats.
You upload it under Settings → Employees, on the apprentice's employment. The card then shows «Seat is free», and your subscription adjusts overnight — anything already paid is credited on your next invoice.
When the apprenticeship ends, the seat counts as normal again. The end date is already on the apprenticeship, so there is nothing to add.