Publish in topic with iot bit 1.6

hi i connect to the public broker (broker.dasboardmqtt.com) with my iot bit 1.6 using microbit but i can´t publish in the topic or subscribe to it
but when i use mqtt dash with my smartphone or with my browser i publish and i subscribe and every thing works well
please can you help me why i don´t succeed to publish …
thx