There are two ways to not use the Sparkplug namespace when publishing messages via Transmission.
- Publish your own free-form messages via Python scripting as noted previously above: MQTT Publishing via MQTT Transmission - MQTT Modules for Ignition 8.x - Confluence
- Use a UNS Transmitter. See here for details: Using MQTT Modules in a UNS Architecture - MQTT Modules for Ignition 8.x - Confluence