All settings that determine how the integration between Younium and Fortnox will work is setup under Settings - Integrations - Fortnox.
The first thing you need to do is to GET STARTED.
After this, you need to log into your Fortnox-account, navigate to your company name drop-down and Administrate Users, there you press ADD INTEGRATION.
This will make a window pop up that you have to fill with the value found in Younium.
After adding the value in Fortnox you will receive an API-code which you put in the Fortnox Authorization Code in Younium. You enter this one in Settings - Integrations - Fortnox by pressing the GET STARTED button.