How can I add a legal age confirmation checkbox to my Buy Button's cart drawer?

[Farhan3]

I have a website on squarespace, in which Shopify buy button is integrated. When I add items to the cart, it opens the cart drawer, I would like add a checkbox text 'I am legal age to buy alcohol', but I don't find any solution to add it on the cart drawer.

I tried to add this code on squarespace in embed script that was provided by Shopify buy button, but it did not work.

          "customCheckbox": {
            "text": "I confirm that I am legal to buy alcohol",
            "required": true
          },