How to send messages to telematics hub from my custom solution via MQTT?

Using MQTT to put the data into your flespi channel.

MQTT usage assumes that you have a good technical background. This may be written on first demand. However, we have everything explained here:

See also
Configuring the telemetry data forwarding from flespi to the Mapon fleet management platform.
Uploading a firmware file to the flespi storage and sending a command to download and apply it.