How to align the axis to a face

I have this shape which is rotated in the scene:
pickpic 2020_045

I want to edit this shape, but to make things easier I would like to align the axis to the front face of the object. I know I can align the camera view to the face (shift numpad 1)…but then the axis are still completely off. Even if I double press the axis they still dont work.

So i would like to create a custom axis orientation, based on the selected face. Can anybody tell me how to do this? I have tried added a transformation orientation with the ‘+’ sign…but nothing seems to happen then…

HI, i think this might help: https://www.youtube.com/watch?v=q_SiM8PIXtI