You can only publish a message to the MQTT Data API to schedule a downlink. But that needs a structured JSON message with Base64 encoded binary data, not some text message like you’re showing. See its documentation. (And never use text messages in LoRaWAN, that’s just a waste of air time.)
So, you cannot use MQTT to simulate a device, if that’s what you’re after. Instead, see How can I test my application by means of seeing the data at my HTTP integration.
Aside, please see How do I format my forum post? [HowTo]