A/B Testing without any access to shopify panel

Hello everyone, i am currently working as a jr. frontend developer in a company and we are giving a/b testing service to our customers.
One of our customers use shopify and we want to make an a/b test.
In our customers theme clicking “Add To Cart” button redirects to the cart page. I want to prevent this behaviour and show a little notification like “Added to Cart!” in a popup so they can continue to shopping. Is it possible to make this happen without accessing the shopify panel? Thanks.