There are three ways to this can be done:

1. The customer can BYO their existing Windows Server license if they purchased Software Assurance (SA) and are eligible for Azure Hybrid benefit (AHUB) use.
2. Procure the Windows Server O.S. license through Server Subscription program by committing for a 1 or 3-year period. 
   Please check the Rhipe FAQ – Windows Server Subscription for more information.
3. Pay on-demand hourly metered rates, which will occur automatically if neither of the above options are used. This option is flexible but not as economical as software subscriptions.