How to enable Home-Assistant MQTT auto-discovery on Tasmota
First, upgrade your Tasmota to the latest firmware. For me, only firmware 10.1.0+ worked with Home-Assistant MQTT autodiscovery.
Then, configure your MQTT server and username/password using the webinterface.
After that, open the console in the webinterface and enter
SetOption19 1
and press return. Now reboot Tasmota using the webinterface.