Tasmota Sonoff Zigbee MQTT OH3

Setup, Configuration and Use Beginners
3

To be sure, please post the output of the Tasmota console, and please use code fences

it depends on your tasmota settings;

Setoption83 
Setoption112 
Setoption100

if all the setoptions are on, the syntax is

cmnd/topic/yourDevice/zbsend

and the Outgoing Value Transformation in your case would be

{"device":"yourDevice", "send":{"Dimmer":"%s"} }