Setting the version of Azure functions from ARM

Need to set the version of Azure functions runtime from an ARM template? Weirdly it is actually an app setting and not something on the site. The variable is called FUNCTIONS_EXTENSION_VERSION and can take on the values ~1 or ~2. In an ARM template it looks like

{
    "type": "Microsoft.Web/sites",
    "kind": "functionapp",
    "name": "[variables('webSiteName')]",
    "tags": {
        "displayName": "Site"
    },
    "apiVersion": "2016-08-01",
    "location": "[resourceGroup().location]",
    "scale": null,
    "resources": [
    {
        "name": "appsettings",
        "type": "config",
        "apiVersion": "2015-08-01",
        "dependsOn": [ "[concat('Microsoft.Web/sites/', variables('webSiteName'))]" ],
        "tags": {
        "displayName": "App settings"
        },
        "properties": {
        "FUNCTIONS_EXTENSION_VERSION": "~2",
        }
    }
    ],...
}