As the title says, I can't figure out how you're supposed to do this. The pricing calculator allows for it, so I'm assuming it's possible.
I've tried:
1) Creating a new cluster
2) Creating a vm and adding it to an existing cluster, then deleting the initial node (Tried with and without the scaleset option)
For #1, I see no options to add a reserved instance during cluster initialization. For #2, I see no options to add an existing vm to an existing aks cluster.
Anyone know how to do this?
After you buy an Azure Reserved Virtual Machine Instance, the reservation discount is automatically applied to virtual machines that match the attributes and quantity of the reservation. A reservation covers the compute costs of your virtual machines. Source
In the documentation you can see that this also applies to AKS.
In other words, you buy a reserved instance and after you create your AKS cluster selecting instances with the same size, the discount will be automatically applied.
A reservation discount applies to the base VMs that you purchase from the Azure Marketplace.
By Marketplace you can also read AKS.