groupTemplate String|Function

The template used to render the groups. By default the widget displays only the value of the group.

<input id="customers" style="width: 400px" />
<script>
    let encode = kendo.htmlEncode;
    $(document).ready(function() {
        $("#customers").kendoDropDownList({
            dataTextField: "ContactName",
            dataValueField: "CustomerID",
            groupTemplate: (data) => `Group: ${encode(data)}`,
            height: 400,
            dataSource: {
                type: "odata",
                transport: {
                    read: "https://demos.telerik.com/kendo-ui/service/Northwind.svc/Customers"
                },
                group: { field: "Country" }
            }
        });
    });
</script>