Can't drag objects into button prefabs to enable OnClick()

I know to enable OnClick() functions I need to drag the object that has the script attached to rather than the script itself into the slot. However, whatever I do, Unity just doesn't let me do it.

Why can't I?

FYI, the buttons are prefabs. I can attach the script to the button prefab and drag the prefab into the slot, but I'm not sure if that's the correct way to do it.

Besides, t$$anonymous$$s is for a text adventure game. So whenever the player chooses an option, it destroys all the other options and instantiate a new set of options according to the scriptable objects that store the dialogue information.