nested-template.json

{
    "$schema": "https://schema.management.azure.com/schemas/2019-04-01/deploymentTemplate.json#",
    "contentVersion": "1.0.0.0",
    "parameters": {
        "StorageAccountName": {
            "type": "String",
            "defaultValue": "storage2020"
        }
    },
    "variables": {
        "templatelink": "C:/Projects/demoARM/templates/newStorageAccount.json"
    },
    "functions": [
        {
            "namespace": "finance",
            "members": {
                "prefixDepartment": {
                    "parameters": [
                    {
                        "name": "resourceName",
                        "type": "string"
                    }
                    ],
                    "output": {
                    "type": "string",
                    "value": "[concat('financeDep-', toLower(parameters('resourceName')))]"
                    }
                }
            }
        }
    ],
    "resources": [
        {
            "apiVersion": "2015-01-01",
            "name": "nestedTemplate",
            "type": "Microsoft.Resources/deployments",
            "properties": {
                "mode": "incremental",
                "templateLink": {
                    "uri": "[variables('templatelink')]",
                    "contentVersion": "1.0.0.0"
                },
                "parameters": {
                    "StorageAccountName": {
                        "value": "[finance.prefixDepartment(parameters('StorageAccountName'))]"
                    }
                }
            }
        }
    ]
}