how to call webmethod from aspx.vb page in vb.net

$(document).ready(function () {
    $('table').delegate('td.EditRow','click',function () {
        var $row = this;
        var userid = $row.id;
        $('#hdnUserId').val(userid);
        $.ajax({
            type: "POST",
            url: "User_Create_Update.aspx/FillUserDetails",
            data: "{'userid': " + userid + "}",
            contentType: "application/json; charset=utf-8",
            dataType: "json",          
            success: function (msg) {
                if (msg.d != '') {
                    $('#txtUserCode').val(msg.d.USRM_USER_CD);
                    $('#txtUserName').val(msg.d.USRM_USER_NAME);
                }
            }
        });
        $('#divPopUp').lightbox_me({
            lightboxSpeed: 500,
            centered: true,
            closeClick: true,
            closeEsc: true
        })
    });
});

<System.Web.Services.WebMethod()> _
<ScriptMethod(ResponseFormat:=ResponseFormat.Json)> _
Public Shared Function FillUserDetails(ByVal userid As String) As DataTable
    Dim dt As DataTable = clsUSRM.GetAllById(userid)
    If dt.Rows.Count > 0 Then
        clsUSRM.USRM_USER_CD = userid
        clsUSRM.USRM_USER_NAME = dt.Rows(1).ToString()

    End If
    Return dt
End Function