Find your newly added application in the integration menu and open it again by clicking on the listed application. Before you can use the Application, Exact Online wants to verify that you as a user are authorized to use the API.
The Authorize button will open a new window, in which you will need to enter your Exact Online login credentials. Afterwards you will need to allow the connection to be created.
NOTE: If you have two-factor authification activated you will see a window to enter the code.
Use your authentication app to retreive the code.
Just enter the code and submit the form. After that you can test the connection.
If the process was successful, the connection can be tested, and will return a green check mark.