According to step 6 at Setting up Home Assistant Assist as default assistant app on an Android phone:
how to do this on a FireOS powered device (Amazon Fire tablets)?
Is this even possible? I did not use Alexa in the past on those devices, now with HA things are finally getting interesting.
(edit: think I solved it on my own. seems to be impossible out of the box, so I used a “Button Mapper” app to trigger Home Assistant Assist when long pressing the volume up button. As my Fire tablet - or it’s Android version - lacks the ability to speak the input so I have to type the text, it’s in the end not useful at all unfortunately… 
)