Customization of Alert popup using HTML

You can use DayPilot.Modal.form() instead to customize the content:
https://api.daypilot.org/daypilot-modal-form/

The following example would display a list:

const form = [
  {text: "Would you like to confirm this list?", type: "html"},
  {html: "<ul><li>item 1</li><li>item 2</li></ul>", type: "html"}
];
const modal = await DayPilot.Modal.form(form);
if (modal.canceled) {
  console.log("canceled");
}
else {
  console.log("confirmed");
}

You can also create the modal dialog content using the UI builder:
https://modal.daypilot.org/builder/

Is there a way to remove the "Cancel" button on that form? I was able to custom build that form to my needs.

At this moment there is no special option for that but you can hide it using CSS:

.modal_default_cancel {
    display: none;
}