Question about Drag and Drop Game and store in List

I am working on a game application where user need to drag and drop prefab into a box and the prefabs are stored in a list. So once user clicked play button, the player will move according to the list of prefabs that being stored. My problem right now is the prefabs that being dragged into a box does not being inserted into a List. I also lack in resources and tutorials because I dont know what is the specific term that suit my concern regarding Drag and Drop in a List.

Can you share resources/tutorials/specific term/tips that related to my game development. I am newbie in unity so most of the term that I used might be wrong. Thank you so much!