The billing account is tied to the user that did the initial checkout and is not connected to your Sync 365 app login.

During your initial check out you would have been requested to create a password and this is for the billing account. If you are unsure of this, please click on “Lost your password” to reset it.

You can access your billing account by going toLogin - Sync 365 (sync365license.com) 

You can also reach this from our website by clicking "S365 App Login> Billing account portal"