<input name="txtAmount" type="text" id="txtAmount" />
<script type="text/javascript" src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.7.1/jquery.min.js"></script>
<script type="text/javascript">
    $(function () {
        $('[id*=txtAmount]').keyup(function (event) {
            // skip for arrow keys
            if (event.which >= 37 && event.which <= 40) { return };
            // format number
            var str = parseFloat($(this).val().replace(/\D/g, "")).toLocaleString('de-DE');
            var pos = str.lastIndexOf('.');
            if (pos == -1) {
                $(this).val(str);
            } else {
                str = str.substring(0, pos) + "," + str.substring(pos + 1);
                $(this).val(str);
            }
            if ($(this).val() == "NaN") {
                $(this).val("");
            }
        });
    });
</script>