Q. How can I create VMs in Azure that take advantage of Hybrid Use Benefit?
A. Hybrid Use Benefit (HUB) is available to customers with Enterprise Agreements and Software Assurance that enables Windows Server licenses on-premises to be leveraged in Azure which results in Windows VMs costing the same as Linux VMs (since there is no charge for the Windows Server license).
To enable the HUB licensing when creating new VMs a licensing type must be specified when creating with PowerShell (-LicenseType Windows_Server) or with JSON templates ("licenseType": "Windows_Server" as part of the properties). This is documented in detail at https://azure.microsoft.com/en-us/documentation/articles/virtual-machines-windows-hybrid-use-benefit-licensing/.
If you have existing VMs in Azure there is currently no way to convert them to HUB licensing via a switch today however this should be enable in the future. For today the only way to achieve this would be to delete the VM while keeping its storage and then create a new VM with the required parameters using the storage from the previous VM.