MQTTBox is a versatile MQTT (Message Queuing Telemetry Transport) client tool that facilitates testing and debugging of MQTT-based applications. MQTT is a lightweight messaging protocol commonly used in IoT (Internet of Things) and other scenarios where low-bandwidth, high-latency, or unreliable networks are a concern.
MQTTBox offers a user-friendly interface that allows users to publish and subscribe to MQTT topics, send and receive MQTT messages, and explore the functionality of MQTT brokers.
How to use it?
Download Link: https://apps.microsoft.com/detail/9NBLGGH55JZG?hl=en-US&gl=US
Make sure you are running the broker on your local machine / on any server.
Open the application, and click on Settings icon. This is the place where you have to set your client. Give a MQTT client name, and under Protocol drop box, select mqtt/tcp as your protocol.
Important: Enter your host. In my case, the mosquitto broker is installed and running in my local machine, and the MQTTT port is 1883. I entered localhost:1883
Publish your Message
Now it’s time to publish the message, and check whether you are receiving at the application.
Things to do: First, it is clear that, the goal is to debug the messages or test your MQTT messages. Verify whether you have successfully connected your broker to the application. If the button is green, and says it is connected. 
Next, click on “Add Publisher” and “Add Subscriber” buttons. Give a random topic name, for instance, “Test”.
Under payload, in the publisher give some random strings to publish. This will be your payload messages.
Click on Publish and Subscriber buttons, to send and receiver messages.
So, the messages are successfully sent and received instantly in the application. Also, from this you can debug or verify the connection with the broker.
You can also send messages from CLI from your local machine. Or from other machine if you are already connected with the same server.
Besides that, you can create multiple clients and server testing.
Alternative for MQTTBox
There are many alternative tools for testing purpose like MQTTX which is maintained by EMQ.
