Send payload with variable over mqtt

Hi Guys,
I got me a radiator thermostat which sends the data via Zigbee. I also use zigbee2mqtt so that I can read and send the data from the radiator thermostat. Normally a string is transmitted via MQTT to the device, which I can enter in the channels under “Custom On/Open Value” to switch on, or “Custom Off/Closed Value” to switch off and this is then published via the topic. According to the documentation of zigbee2mqtt the command to set a new temperature is the following: {“current_heating_setpoint”: VALUE}
Since instead of the word VALUE, of course there must be a value, I don’t know what to do. Therefore the question, how can I define a variable that gets the value by setting a setpoint, so that I can send it as a string to the corresponding topic via MQTT.

The radiator thermostat is the GS361A-H04 from Unitec.

Edit: Maybe for a better explanation.
I have these two topics:
MQTT State Topic: zigbee2mqtt/Heizung_Wohn_klein/current_heating_setpoint
MQTT Command Topic: zigbee2mqtt/Heizung_Wohn_klein/set

And i want to send the command: {“current_heating_setpoint”: VALUE}
to the topic: zigbee2mqtt/Heizung_Wohn_klein/set
And the Value should be replaced with a Setpoint value from the channel

Format before publish

{"current_heating_setpoint": %s}

It was really that easy… Thanks a lot!