Hi,
actually i try to define a MQTT Thing through config files. Incoming Updates are working finde, but at some Points i need to set States vie MQTT.
The config looks like:
Type number : goeCharger_Ampere “goe Ampere Soll” [ stateTopic=“go-eCharger/000xxx/status”, commandTopic=“go-eCharger/000xxx/cmd/req”, transformationPattern=“JSONPATH:$.amp”]
but I have to set a prefix for the Outgoing Messages: “amp=”. Where should this be defined?
Please help.
Thanks & Greetings
vlamers
You want this: https://www.openhab.org/addons/bindings/mqtt.generic/#format-before-publish.
The correct key to be used in a textual config file can be seen here: https://www.openhab.org/addons/bindings/mqtt.generic/#channel-configuration
As you can see it helps a lot by studying the binding documentation 
i searched for hours but didnt find this documentation.
Thanks for your help and sorry for the question!
Unfortunately my mqtt command is still not working.
My MQTT Thing looks like:
Type number : goeCharger_Ampere "goe Ampere Soll" [ stateTopic="go-eCharger/000xxx/status", commandTopic="go-eCharger/000xxx/cmd/req", formatBeforePublish="amp=%s", transformationPattern="JSONPATH:$.amp"]
And the MQTT Command has to look like:
mosquitto_pub -h localhost -p 1883 -u xxx-P yyy-t 'go-eCharger/000xxx/cmd/req' -m amp=16
Where is my fault?
Thx!
I got it running with mqtt, but setup wit PaperUI. Did you check with an mqtt client what openhab finally posts on this channel?