Taints can be setup with the [--node-taints] flag only when you are adding a node pool with az aks nodepool add command:

Add a node pool to the managed Kubernetes cluster.

az aks nodepool add --cluster-name
                    --name
                    --resource-group
                    [--node-taints]

However, you cannot add taints to already existing node pool:

You can't change the node taints through CLI after the node pool is created.

A very similar topic is being discussed in this open thread.

So currently it is not possible to set taints to an existing node pool on AKS. But you can set them up while adding a new node pool to the managed cluster.

If you provisioned your cluster with Terraform, you can specify node_taints in azurerm_kubernetes_cluster_node_pool see here However, you can't add taints to default node pool