Enabling button after 100 clicks

so I wanted to create a secret or something in my game. Basically what i wanted to ask is how do you make a button work after clicking it a 100 times. After clicking a 100 times it should give me 100 money. Im new to unity so it would be nice if you could explain it more precise. I tried this code:

using System.Collections;
using System.Collections.Generic;
using TMPro;
using UnityEngine;

public class SecretButton : MonoBehaviour
{

    public int clickcounter = 0;
    public TMP_Text scoreText;

    int moneyAmount;

    void Start()
    {

    }

    void Update()
    {
        if (clickcounter == 100)
        {
            moneyAmount += 100;

            PlayerPrefs.SetInt("MoneyAmount", moneyAmount);

            moneyAmount = PlayerPrefs.GetInt("MoneyAmount");

            scoreText.text = moneyAmount.ToString();
        }
    }

    public void increaseClicks()
    {
        clickcounter++;
    }
}

but it started giving me 100 money all the time, but i wanted it to give me only once 100 money.

Easiest way I can think of is creating a variable _counter assigned to the button and increase it by one for each button press.
inside the button press function, include an if statement that checks whether the _counter>100 and in that case it starts the followup actions, otherwise it skips the action.

The answer is quite high level, but if you want more details, you have to share more details about what you are trying to accomplish

update:

I can see in your edit the code now, your additional problem with giving 100 money multiple times is because you don't make this function execute only once.
You can solve this by adding another variable _secret_available and initiate it as True, when the function is activated, set the variable to False.

You also need to edit the if clause to be like this:

if(counter ==100 && _secret_available)

Also my suggestion would be to replace counter==100 with counter >=100 so that you don't have possible bugs in case the counter jumps from 99 to 101 without passing by 100. Might not be a problem in the moment but I believe it is more robust