POST to an external API

I would like to calculate number of trees to be planted per transaction and POST to https://offset.earth/ API. Would also like to show to customers in the order confirmation how many trees they planted for this transaction.

would anyone know how to achieve this in squarespace?

Thanks in advance

<script>
    var xhttp = new XMLHttpRequest();
        xhttp.onreadystatechange = function () {
                if (this.readyState == 4 && this.status == 200) {
                    alert(this.responseText);
                }
            };
            xhttp.open("POST", "TARGET URL", true);
            xhttp.setRequestHeader("Content-Type", "application/json");
            xhttp.setRequestHeader("Authorization", "AUTHORISATION ID);
              var num =  1;  //send a random number
            xhttp.send('{"number": ' + num + '},{"test": true}');
  
</script>

I managed to figure this out with help. Hope this will help someone. I injected the above code to order confirmation.