How to load user controls dynamically on selectedIndexChanges?

On SearchEnquiry.ascx (userControl)

On SearchEnquiry.ascx (userControl)

<telerik:RadComboBox ID="cmbSearchFor" runat="server" Width="178px" AutoPostBack="true"

                OnSelectedIndexChanged="cmbSearchFor_SelectedIndexChanged">

                <CollapseAnimation Duration="200" Type="OutQuint" />

                <Items>

                    <telerik:RadComboBoxItem Text="Select" Value="None" Selected="true" />

                    <telerik:RadComboBoxItem Text="Test" Value="Test" />

                    <telerik:RadComboBoxItem Text="Inquiry" Value="Enquiry" />

                    <telerik:RadComboBoxItem Text="Complaint" Value="Complaint" />

                    <telerik:RadComboBoxItem Text="Home Collection" Value="HC" />

                    <telerik:RadComboBoxItem Text="TeleRegistration" Value="TR" />

                Items>

            telerik:RadComboBox>

 

 

   <asp:PlaceHolder ID="PlaceHolder1" runat="server">asp:PlaceHolder>

 

protected void cmbSearchFor_SelectedIndexChanged(object o, Telerik.Web.UI.RadComboBoxSelectedIndexChangedEventArgs e)

    {

 

        UserControls_SearchTest ucSearchTest = (UserControls_SearchTest)Page.LoadControl("UserControls/SearchTest.ascx");

        PlaceHolder1.Controls.Add(ucSearchTest);      

    }

Im using this code, but it not show the usercontrols.