Sorry, I thought you talk about notifications.
If you run an auto-discovery, names for devices and service URLs will be created automatically. Thus, you will not need any placeholders. Create an auto-discovery group and specify the settings suitable to your needs. In section Name Resolution, you can define whether to monitor newly discovered devices via their DNS, WMI, or SNMP names or via IP address which will then appear automatically as "device name".
See PRTG Manual: Auto-Discovery for details.