Hi mswahba,
Check this example. Now please take its reference and correct your code.
HTML
<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="cc1" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<title></title>
<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.9.1/jquery.min.js"></script>
<link rel="stylesheet" type="text/css" href="https://cdnjs.cloudflare.com/ajax/libs/select2/4.0.3/css/select2.min.css" />
<script type="text/javascript" src="https://cdnjs.cloudflare.com/ajax/libs/select2/4.0.3/js/select2.min.js"></script>
<script type="text/javascript">
$(function () {
$(".js-example-programmatic").select2({ placeholder: "Select" });
$(".js-example-placeholder-single").select2({
placeholder: "Select",
allowClear: true
});
});
</script>
</head>
<body>
<form id="form1" runat="server">
<div>
<cc1:ToolkitScriptManager runat="server">
</cc1:ToolkitScriptManager>
<cc1:TabContainer ID="UserProfile" runat="server">
<cc1:TabPanel ID="tpnlEmail" runat="server" HeaderText="Tab 1">
<ContentTemplate>
<select multiple style="width: 300px" class="js-example-programmatic">
<option value="Alice Mutton">Alice Mutton</option>
<option value="Aniseed Syrup">Aniseed Syrup</option>
<option value="Boston Crab Meat">Boston Crab Meat</option>
<option value="Camembert Pierrot">Camembert Pierrot</option>
<option value="Carnarvon Tigers">Carnarvon Tigers</option>
<option value="Chai">Chai</option>
<option value="Chang">Chang</option>
<option value="Chartreuse verte">Chartreuse verte</option>
</select>
</ContentTemplate>
</cc1:TabPanel>
<cc1:TabPanel ID="tpnlAddress" runat="server" HeaderText="Tab 2">
<ContentTemplate>
<asp:DropDownList ID="ddl1" multiple="multiple" Width="300px" runat="server" CssClass="form-control js-example-placeholder-single">
<asp:ListItem Text="Alice Mutton" Value="Alice Mutton"></asp:ListItem>
<asp:ListItem Text="Aniseed Syrup" Value="Aniseed Syrup"></asp:ListItem>
<asp:ListItem Text="Boston Crab Meat" Value="Boston Crab Meat"></asp:ListItem>
<asp:ListItem Text="Camembert Pierrot" Value="Camembert Pierrot"></asp:ListItem>
<asp:ListItem Text="Carnarvon Tigers" Value="Carnarvon Tigers"></asp:ListItem>
<asp:ListItem Text="Chai" Value="Chai"></asp:ListItem>
<asp:ListItem Text="Chang" Value="Chang"></asp:ListItem>
<asp:ListItem Text="Chartreuse verte" Value="Chartreuse verte"></asp:ListItem>
</asp:DropDownList>
</ContentTemplate>
</cc1:TabPanel>
</cc1:TabContainer>
</div>
</form>
</body>
</html>
Screenshot
