To connect to our API, whether that is from your own applications/scripts or from a third party plugin like WHMCS or PowerPanel, use https://api.openprovider.eu.
The username is one of the users in your account, the corresponding password is valid for the API as well. You can log in with a hash value instead of a password. To get the hash value for a password, go to Account>Security>Password. When you change your password, a new hash value will be generated for you.
For information about the hash value and further security measures in Openprovider, please refer to our corresponding blog post.