Unable to assign VMs to azurerm_lb_backend_address_pool - azure

Question

I was able to create 2 Linux VM's in azurerm_availability_set and now would like to attach these VM's to azurerm_lb_backend_address_pool but other than the options listed below in my code. I don't see an availability set option but when I goto the Azure portal I see an availability set option through the portal. Not sure if I'm doing something wrong here.
Please review the code below and let me know as of where can I add availability set option. So that I can attach the 2 VMs.
resource "azurerm_lb_backend_address_pool" "backend_pool" {
resource_group_name = "${azurerm_resource_group.test.name}"
loadbalancer_id = "${azurerm_lb.lb.id}"
name = "webBackendPool"
}

Answer 1

Assigning VMs in the load balancer backend pool is actually to assign the network interfaces of VMs to the backend pool, so you could use azurerm_network_interface_backend_address_pool_association resource to binds the NICs of VMs to a backend pool.
For example,
...
resource "azurerm_network_interface" "test" {
name = "${var.prefix}-nic"
location = "${azurerm_resource_group.test.location}"
resource_group_name = "${azurerm_resource_group.test.name}"
ip_configuration {
name = "testconfiguration1"
subnet_id = "${azurerm_subnet.internal.id}"
private_ip_address_allocation = "Dynamic"
public_ip_address_id = "${azurerm_public_ip.test.id}"
}
}
...
resource "azurerm_network_interface_backend_address_pool_association" "test" {
network_interface_id = "${azurerm_network_interface.test.id}"
ip_configuration_name = "testconfiguration1"
backend_address_pool_id = "${azurerm_lb_backend_address_pool.backend_pool.id}"
}
resource "azurerm_lb" "lb" {
name = "weblb"
resource_group_name = "${azurerm_resource_group.test.name}"
location = "${azurerm_resource_group.test.location}"
sku = "${var.lb_sku}"
frontend_ip_configuration {
name = "${var.frontend_name}"
subnet_id = "${azurerm_subnet.frontend.id}"
private_ip_address = "10.0.1.10"
private_ip_address_allocation = "Static"
}
}
resource "azurerm_lb_backend_address_pool" "backend_pool" {
resource_group_name = "${azurerm_resource_group.test.name}"
loadbalancer_id = "${azurerm_lb.lb.id}"
name = "webBackendPool"
}