This repo should help you host an MQTT server on your laptop and connect an esp32 to the MQTT server, publish and subscribe to topics, etc.
install mosquitto in your system.
Then for the localhost setup, we need to create a configuration file. create a file and add .conf as the extension.
inside the .conf file, paste the following lines:
listener 1883 127.0.0.1
allow_anonymous true
After you've pasted the above lines, you can start the mosquitto service by this command:
sudo systemctl status mosquitto
and then you can either publish or subscribe to topics.
In order to publish, you can use the following command:
mosquitto_pub -h Your_Laptop's_IP -t topic_name -m "enter message here"
that's the steps you need to follow in order to publish a message through mosquitto.
you can try the example code here.