Where to place new AJAX code in theme files for recharge checkout integration?

[biznazz101]

Hello, I am updating a store from the recharge checkout to recharge shopify checkout integration. I have been told that when the conversion takes place the current AJAX parameters will need to be updated along with widget customizations. I have reviewed this guide but am a little out of my depth.

It shows this code will need to be used, my question is where do I place this code in my theme files? I don't see the AJAX code I will need to replace:
STORE URL: hairtieclub.com

New AJAX Code:

<input type="button" id="add-to-cart-button" value="ADD TO CART" onClick="addItemToCart(36807328039071, 1, {{product.selling_plan_groups[0].selling_plans[0].id}})">       
<script>  
function addItemToCart(variant_id, qty, selling_plan) {

    data = {
      "id": variant_id,
      "quantity": qty,
      "selling_plan": selling_plan
    }
    jQuery.ajax({
      type: 'POST',
      url: '/cart/add.js',
      data: data,
      dataType: 'json',
      success: function() { 
        window.location.href = '/cart'; 
      }
    });
  }
</script>    

 Current Page:

The theme files I see that seem to be like they could be relevant are:
snippets/ajax-cart-template.liquid
assets/ajax-cart.js.liquid
templates/page.sign-up.liquid
snippets/subscription-product.liquid

I cannot seem to figure out where I will be putting this new code, or how it will interact with the rest of the theme files and help would be appreciated, thanks guys!