Send KNX telegram "block" when a Tasmota device (MQTT) becomes unavailable


I need help in setting this up -

When a Tasmota device is available on network, HA sends a KNX telegram, which will be used by my other visualization to enable or disable in the visualization. (I think its called block in KNX terminology)

Thanks to Mathias - I have learnt to send/recv the KNX telegrams using state changes, but this one is getting me stumped. How do I use LWT (MQTT marks LWT as online or offline depending on the device availability) as trigger to send a KNX telegram. I cannot see LWT as being exposed in HA.

Thank you

Answering to myself -