You first need to login to WHMCS to accomplish this task.
1. Hover on “Setup” on the menu tabs at the top, and navigate to “Products/Services“, hover on it and another submenu will appear. Select “Product Addons“.
2. Click on the “Add New Addon” button.
3. It’s now time to customize the product addon, set its pricing and choose associated products and downloads, if any.
4. Save Changes.