Access the Swagger directly from the Clearing UI by clicking on the rocket icon on the top bar
A new tab opens and you are now in the Swagger UI were you can try all API calls available from the Clearing UI
Authorization
First click on the exclamation mark, tick the swagger box and then click the Authorize button (as shown below)
Further documentation can be found in the Nord Pool Developers Portal: https://developers.nordpoolgroup.com/reference/cass-introduction