add custom attribute to kendo combo box

How to add custom attribute to kendo combo-box?

HTML code-sample

<input id="cbbUsers" class="form-control"/>

JavaScript code-sample

<script type="text/javascript">

//BIND DEFAULT KENDO COMBOBOX.

    $("#cbbUsers").kendoComboBox({

        autoBind: true,

        cascadeFrom: "categories",

        filter: "contains",

        placeholder: "Select users...",

        dataTextField: "UserName",

        dataValueField: "UserId",

        select: onSelectUser,

        dataSource: {

            data: "api/users/GetUsers"

        }

    }).data("kendoComboBox");

  //OnSelect EVENT USER FUNCTION.

    function onSelectUser(e) {

        var oldVal= $("#cbbUsers").val();

        $("#cbbUsers").closest(".k-combobox").attr("oldVal", oldVal);

    };

</script>