In order to use the APIs you will need a token: this can be created from the account dashboard of any paid plan. Just copy the token value as a UUID directly from the web interface, it should be something like 4333beb0-5506-438d-b590-6a3a3094adfbfor example.
Once you have a token, you will need to authorise API calls using your token. Access the Swagger API page and click on the green "Authorize" button and enter, as an API key, the string "token" followed by a space and your token, like in this example:
API authorisation screen
Once you enter the token, just click on "Authorize" and, from now on, all the calls made through the interface will be executed using your account.