After you have subscribed to the Business plan, will need to first create a team account.
Then, you can generate an API token by going to your 'Team' page and scrolling to the bottom:
If you have generated an API token, it will either be automatically be deducted from your prepaid bundle or, if you have pay-as-you-go, it will be registered in that way for €/$0.50 per document.
It also works in the same way with multiple teams and multiple API tokens.