You will need to authenticate with the account you used to place the initial order.
  

1. Go to the Microsoft 365 admin center: https://admin.microsoft.com
2. Authenticate using your Microsoft 365 account (make sure it's the one used for the original order).
3. Under Billing, then Your products, click on your Waldo subscription.
4. Then click on Buy licenses or Remove licenses.

Here are two articles you may find interesting as well:

• How to subscribe to the complete version with or without the 30 days trial.
• How to assign your licenses.