You can’t update/change the value of a dummythermostat through the API. But you can update (user)variables). Just chang one variable over API and let a rule kick in to take that value and set it to the thermostat setpoint.
Sorry … was wrong, you can update a setpoint. You need to GET to : http://IP/api/device/thermostat/changeTemperatureTo?temperatureSetpoint=$temperature you can use my previous example to contact API from a rule.