The loadingHtml property (string) specifies HTML displayed in the modal dialog before the target URL loads when using DayPilot.Modal.showUrl().
DayPilot.Modal.loadingHtmlnullThis temporary content is shown only while the page requested by DayPilot.Modal.showUrl() is loading.
const modal = new DayPilot.Modal({
loadingHtml: "<div>Loading...</div>",
// ...
});
modal.showUrl("/dialog.html");