How to multiple select item in ListBox without press CTRL in Asp.Net?

Ref: Making a standard ASP.NET listbox do multiselect without holding Ctrl

Just change your listbox to

<asp:ListBox ID="ListBox1" runat="server" Height="210px" Width="203px" SelectionMode="Multiple" onclick="ListBoxClient_SelectionChanged(this, event);">
        <asp:ListItem>1000</asp:ListItem>
        <asp:ListItem>2000</asp:ListItem>
        <asp:ListItem>4000</asp:ListItem>
        <asp:ListItem>4000</asp:ListItem>
        <asp:ListItem>5000</asp:ListItem>
        <asp:ListItem>6000</asp:ListItem>
</asp:ListBox>

And add following script in your <script> tag

<script type="text/javascript" language="javascript">
        var selectedClientPermissions = [];
        function pageLoad() {
            var ListBox1 = document.getElementById("<%= ListBox1.ClientID %>");
            for (var i = 0; i < ListBox1.length; i++) {
                selectedClientPermissions[i] = ListBox1.options[i].selected;
            }
        }
        function ListBoxClient_SelectionChanged(sender, args) {
            var scrollPosition = sender.scrollTop;
            for (var i = 0; i < sender.length; i++) {
                if (sender.options[i].selected) selectedClientPermissions[i] = !selectedClientPermissions[i];
                sender.options[i].selected = selectedClientPermissions[i] === true;
            }
            sender.scrollTop = scrollPosition;
        }
</script>