How to enable a joystick on a prefab?

I am stuck because I want to enable a joystick on a prefab but i cant put the joystick on something what is in the Project. How can I fix that?
(btw. I cant search for a GameObject because it isn´t one :D),I am stuck because I want to enable a joystick on a prefab but i cant put the joystick on something what is in the Project. How can I fix that?
(btw. I cant Seach foe GameObject because it isn´t one :D)

I answered in another forum post:
https://answers.unity.com/questions/1782756/how-to-attach-on-screen-joystick-prefab-to-gameobj.html?childToView=1858083#answer-1858083
Let me know if it works!