Sync an attribute's value with MQTT

I’m using the ‘door’ asset which comes with a ‘status’ boolean attribute. I’m able to receive its status by subscribing to external Mqtt broker, this works fine. Now I want to change the status of the door in openremote by toggling the same boolean attribute, which should then publish this change to the external Mqtt broker. How can this be done? I tried adding the ‘publish topic’ to the config below, but it doesn’t send anything. The error log suggests that I cannot choose my own publish topic: