resourceGroup-in-vars.json

{
    "$schema": "https://schema.management.azure.com/schemas/2019-04-01/deploymentTemplate.json#",
    "contentVersion": "1.0.0.0",
    "resources": [],
    "variables": {
        "value": "[resourceGroup()]"
    },
    "outputs": {
        "resourceGroupOutput": {
            "value": "[variables('value')]",
            "type" : "string"
        }
    }
}