Add licenses to a subscription purchased through the Volume Licensing Service Center
Licenses for Office 365 Business Essentials, Office 365 Business, Office 365 Business Premium, and the Office 365 Enterprise plans need to be purchased through the third party partner that you originally purchased Office 365 from. If the partner that the subscription was purchased from is known by Microsoft, you can find out how to contact your partner using the following procedure.
In the Admin center, go to the Subscriptions page, or choose Billing > Subscriptions.
If you're using Office 365 Germany, go to this Subscriptions page.
If you're using Office 365 operated by 21Vianet, go to this Subscriptions page.
On the Subscriptions page, select the subscription to which you want to add licenses, then choose the Volume Licensing Service Center (VLSC) link.
Follow the steps in the VLSC to complete your purchase.