Hello there.

This falls under custom work, so I can only offer some advice about this. Please take a look at this example:

<script>
  var getUrlParams = function (url) {
    var params = {};
    var parser = document.createElement('a');
    parser.href = url;
    var query = parser.search.substring(1);
    var vars = query.split('&');
    for (var i = 0; i < vars.length; i++) {
      var pair = vars[i].split('=');
      params[pair[0]] = decodeURIComponent(pair[1]);
    }
    return params;
  };
  window.urlParams = getUrlParams(window.location.href);
  window.beforeConfirmBookingLoaded = function () {
    var firstName = document.querySelector("#am-confirm-booking > div:nth-child(1) > form > div.am-confirm-booking-data.el-row > div:nth-child(3) > div > div > div > input");
    var lastName = document.querySelector("#am-confirm-booking > div:nth-child(1) > form > div.am-confirm-booking-data.el-row > div:nth-child(4) > div > div > div > input");
    if ('firstName' in window.urlParams) {
      firstName.value = window.urlParams.firstName;
      firstName.dispatchEvent(new Event('input'));
    }
    if ('lastName' in window.urlParams) {
      lastName.value = window.urlParams.lastName;
      lastName.dispatchEvent(new Event('input'));
    }
  }
</script>

It uses our JS hook to modify the value of first name and last name through URL, or to use our email notification placeholders in the URL.

For example:

https://localhost/ameliafull/index.php/booking/?firstName=John&lastName=Doe - this will add "John" in the First Name field, and "Doe" in the Last Name field.

https://localhost/ameliafull/index.php/booking/?firstName=%customer_first_name%&lastName=%customer_last_name% this will add both the name and the last name of the customer, pulling it from our database if they're logged in.

You can modify the JS to do something similar with custom field values. You can target the class "cf-ID".