checkout-subscribe.html

<script>
  document.addEventListener("DOMContentLoaded", function() {
    $("button[type='submit']").click(function(e) {
      if ($("#subscribeToNewsletter").is(':checked')) {
        e.preventDefault();
         
        // grab fields
        var email = $('input#shippingEmail').val();
        var firstname = $('input#shippingFirstName').val();
        var company = $('#shippingCompanyName').val() || "";
        var lastname = $('input#shippingLastName').val();
        var address = $('input#shippingAddressOne').val();
        var address2 = $('input#shippingAddressTwo').val();
        var city = $('input#shippingAddressCity').val();
        var state = $('#shippingState').val();
        var zip = $('#shippingZipCode').val();
 
        var settings = {
          "async": true,
          "crossDomain": true,
          "url": "https://manage.kmail-lists.com/ajax/subscriptions/subscribe",
          "method": "POST",
          "headers": {
            "content-type": "application/x-www-form-urlencoded",
            "cache-control": "no-cache"
          },
          "data": {
            "g": "LIST_ID",
            "email": email,
            // pass in additional fields
            "$fields": "$source, $first_name, $last_name $address1 $address2 $city $region $zip $organization",
            "$source": "Checkout Page",
            "$first_name": firstname,
            "$last_name": lastname.
            "$address1": address,
            "$address2": address2,
            "$city": city,
            "$region": state,
            "$zip": zip,
            "$organization": company
          }
        };
        $.ajax(settings).done(function(response) {
          console.log(response);
          // select and submit form after subscribing
          $("#dwfrm_shipping").submit();
        })
      }
     })
  });
</script>