CreateCustomID.html

<script src="//code.jquery.com/jquery-1.11.1.min.js" type="text/javascript"></script>
<script src="//cdnjs.cloudflare.com/ajax/libs/jquery.SPServices/2014.01/jquery.SPServices.min.js" type="text/javascript"></script>
<script type="text/javascript">
var count;
var itemId;
$(document).ready( function () {
    var numStr;
    $().SPServices({
        operation: "GetListItems",
        async: false,
        listName: "counter",
        completefunc: function (xData, Status) {
            $(xData.responseXML).SPFilterNode("z:row").each( function () {
                itemId= $(this).attr("ows_ID");
                count = Number($(this).attr("ows_Title"));
            });
        }
    });

    count++;
    numStr = "CustomID - " + ("0000000" + count).slice(-8);

    $("input[title='CustomID']").val(numStr).attr("readonly", "readonly").css("color", "#6a6a6a");
    $(":text:not([readonly])").eq(0).focus();

});

function PreSaveAction(){
    $().SPServices({
        operation: "UpdateListItems",
        async: false,
        batchCmd: "Update",
        listName: "counter",
        valuepairs: [["Title", count]],
        ID: itemId,
        completefunc: function(xData, Status) {
        }
    });
    return true;
}
</script>