You do not explicitly specify a reservation when provisioning a VM in Azure. Instead, Azure automatically applies the reservation benefit to any matching VM instance within the scope of the reservation (e.g., subscription, resource group, or shared). The reservation is applied based on key attributes such as region, VM size, and operating system type. As long as your VM configuration matches an available reservation and is within the correct scope, the discount is automatically applied—there’s no need to manually link the reservation during VM creation.
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hth
Marcin