I use a app called Shogun to build my landing page.
I sell a clothing product that has multiple sizes and colors.
When I click the ATC button in the collection page in my landing page, it automatically add one unit of size XS to the cart of that product. The problem here is that you can't choose the size you want and you can't choose the quantity.
What I want to do is this: [login to view URL]%d7%a0%d7%a9%d7%99%d7%9d
Try and click the ATC button.
As you can see, a page will pop up that allow you to choose whichever size you want, also to choose the quantity, also it provide you with additional information about the product, and then you can add to cart.
I want to do that on my ATC button in my landing page's collection section.
Again, I use Shopify and I use Shogun app to build my landing page.
If you are able to do this pop up page, contact me.