How to add a code in html/javascript to a site

I'm trying to add a shop section in my site, but not just a regular one, like you can choose ur preferences and they will make the price (either higher or lower), for example, you can choose to add certain features to your order, etc. i want to add this https://elo-boost.net/boosting/purchase/ into my site (You choose your game rank and your desired one, i'm asssuing this is some sort of algorithm). the problem is i dont know how to add it to my site, im using WebStarts.com editor, i can Edit Source of the html and i can paste code (html or javascript). Assuming i have the files of the site ( https://elo-boost.net/boosting/purchase/ ) how do I copy that section and add it to my site (I will edit it obviously, won't be the same on my site).