That is one of they many ways in which you can add money to a Mullvad account. You should try creating an account; it only takes one click.
https://mullvad.net/en/account/create/
That link automatically creates a new account. No names, no email address (except if you set up a PayPal subscription), not even a password. Your account number is literally all you need. They even have a FAQ that provides a generalized database schema.
https://mullvad.net/en/help/no-logging-data-policy/#numbered