MQTT Auto Discovery - no results

I’m trying to send some external data to Home Assistant via MQTT. I’m afraid I have reached a wall - I can’t get the MQTT discovery to create the entities.

I’m back to just a simple Mosquitto client on the remote machine for testing. When I run the below, I see a connection in the Mosquitto log, but no entity in HA or device in the integration:
mosquitto_pub -h homeassistant.mydomain -u xxxxx -P xxxxxxx -t "homeassistant/sensor/potatosensor/config" -m '{"state_topic": "homeassistant/sensor/potatosensor/state", "name": "potato sensor"} '


[02:21:17] INFO: [INFO] found xxxxx on local database
1619504477: New client connected from as gas-water-RTL (p2, c1, k60, u'xxxxx').
1619504477: Client gas-water-RTL disconnected.

Thoughts? This is on Hass on Raspberry Pi.