To use the Billing Addon (or other 3rd party Addon), you need to create API access tokens.
First step is as an admin to allow access of the 3rd party App to the Panel.
This is done by creating an API Access Token.
Token has name/desc, expiration, status, scope and IP whitelist.
From the Key(Credentials) button, you can see the generated token value.
Next step is as a user to allow the App to perform actions from your account.
This can be done from user's profile page or edit page.
For the Billing Addon to work, user and token need to have the following permissions:
Once token is created, you can use the value in App.
Since this is credentials, they have to be kept secret.