Check out example codes for "unity assign button onclick". It will help you in understanding the concepts better.

Code Example 1

public Button yourButton;	

void Start () 
{
	Button btn = yourButton.GetComponent<Button>(); //Grabs the button component
	btn.onClick.AddListener(TaskOnClick); //Adds a listner on the button
}	
void TaskOnClick()
{
	Debug.Log ("You have clicked the button!");
}

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