Shopify Discout code Autofill via URL

getDiscountCode.html

<script>
  /* Put this in theme.liquid, preferably right before "</body>" */
  (function() {
    var discountParam = document.location.search.match(/discount=(\w+)/);
    if (discountParam && discountParam.length > 1) {
      document.cookie = discountParam[0];
    }
  })();
</script>

setDiscountCode.html

<script>
  /*
    Put this in cart.liquid, preferably at the bottom of the file.
    Also, make sure your cart's "<form>" has an ID of "cartform".
  */
  (function() {
    var discountCookie = document.cookie.match(/discount=(\w+)/);
    if (discountCookie && discountCookie.length > 1) {
      document.getElementById('cartform').action = '/cart?' + discountCookie[0];
    }
  })();
</script>

more information https://ecommerce.shopify.com/c/shopify-discussion/t/autofill-discount-via-url-a-step-by-step-walkthrough-eg-auto-apply-discount-code-using-a-link-266537