Configure_DSC_Extension_To_Onboard_To_AzureAutomationDSC.tf

variable "dsc_key" {
  default = "dsc_registration_key_from_portal"
}

variable "dsc_endpoint" {
  default = "dsc_registration_url_from_portal"
}

variable dsc_config {
  default = "node_configuration_you_want_applied__can_leave_blank"
}
variable dsc_mode {
  default = "applyAndMonitor"
}

resource "azurerm_virtual_machine_extension" "dsc" {
  name                 = "DevOpsDSC"
  location             = "${var.location}"
  resource_group_name  = "${var.resource_group_name}"
  virtual_machine_name = "${var.vm_name}"
  publisher            = "Microsoft.Powershell"
  type                 = "DSC"
  type_handler_version = "2.73"
  depends_on           = ["azurerm_virtual_machine.virtual_machine"]

  settings = <<SETTINGS
        {
            "WmfVersion": "latest",
            "ModulesUrl": "https://eus2oaasibizamarketprod1.blob.core.windows.net/automationdscpreview/RegistrationMetaConfigV2.zip",
            "ConfigurationFunction": "RegistrationMetaConfigV2.ps1\\RegistrationMetaConfigV2",
            "Privacy": {
                "DataCollection": ""
            },
            "Properties": {
                "RegistrationKey": {
                  "UserName": "PLACEHOLDER_DONOTUSE",
                  "Password": "PrivateSettingsRef:registrationKeyPrivate"
                },
                "RegistrationUrl": "${var.dsc_endpoint}",
                "NodeConfigurationName": "${var.dsc_config}",
                "ConfigurationMode": "${var.dsc_mode}",
                "ConfigurationModeFrequencyMins": 15,
                "RefreshFrequencyMins": 30,
                "RebootNodeIfNeeded": false,
                "ActionAfterReboot": "continueConfiguration",
                "AllowModuleOverwrite": false
            }
        }
    SETTINGS

  protected_settings = <<PROTECTED_SETTINGS
    {
      "Items": {
        "registrationKeyPrivate" : "${var.dsc_key}"
      }
    }
PROTECTED_SETTINGS
}

The one thing i would like to add is the following:
depends_on = ["azurerm_virtual_machine.virtual_machine"] is only if you are creating a VM at the same time of adding the extension,
type_handler_version = "2.77" is the latest DSC --ver
and in the variable dsc_config { } it is better to specify a config name cos it defies the purpose