Buy reserved capacity
* Sign in to the Azure portal.
* Select All services > Reservations.
* Select Add and then in the Purchase Reservations pane, select SQL Database to purchase a new reservation for SQL Database.
* Fill in the required fields. Existing databases in SQL Database and SQL Managed Instance that match the attributes you select qualify to get the reserved capacity discount. The actual number of databases or managed instances that get the discount depends on the scope and quantity selected.

* Review the cost of the capacity reservation in the Costs section.
* Select Purchase.
* Select View this Reservation to see the status of your purchase.
Reference:
https://docs.microsoft.com/en-us/azure/azure-sql/database/reserved-capacity-overview