Populate textboxes in a popup using a dropdown

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ASP.NET

Ajax

I have a popup that contains a dropdown list and few textboxes. When I click the dropdown it should populate the textboxes inside the popup but the popup disapears.
How can i solve this by not making my dropdown to be AutopostBack false?

Below is my panel with dropdown and textbox.

<asp:Panel ID="PNL" runat="server" style="display:none; width:900px; background-color:White; border-width:2px; border-color:Black; border-style:solid; padding:20px; ">
                    <%-- Form to finalize appointment--%>

  <table id="Table1" runat="server">
   <tr>
   <td>
   Select Patient
   </td>
    <td style="width: 166px">
        <asp:DropDownList ID="ddlPatientID" runat="server"
            AutoPostBack="true"
              onselectedindexchanged="ddlPatientID_SelectedIndexChanged"
            ForeColor="Black"   height="22px" Width="162px"
            style="border:1px inset black" >

        </asp:DropDownList>
   </td>
   </tr>
       <tr>
           <td>
               Name</td>
           <td style="width: 166px">
               <asp:TextBox ID="txtViewPatientName" runat="server"
                   ForeColor="Black" ReadOnly="True" CssClass="textbox"></asp:TextBox>
           </td>

Below I have a column that is inside gridview. When I click the LinkButton, the Panel with the dropdown and textbox will apear.

<Columns >

             <asp:TemplateField HeaderText="Create Appointment">

           <ItemTemplate>
               <asp:LinkButton ID="createAppointment" runat="server"  OnClick="createAppointment_Click" OnClientClick='<%# "frmFindAppointment.aspx?Fullname=" + HttpUtility.UrlEncode(Eval("Fullname").ToString())+"&RecordNumber="+ HttpUtility.UrlEncode(Eval("RecordNumber").ToString())+"&Date="+  HttpUtility.UrlEncode(Eval("Date").ToString())+"&SlotID="+ HttpUtility.UrlEncode(Eval("SlotID").ToString())+"&Timeslot="+ HttpUtility.UrlEncode(Eval("Timeslot").ToString())  %>' Text="Create Appointment" />
            <ajaxToolkit:ConfirmButtonExtender ID="ConfirmButtonExtender2"  runat="server"
                       TargetControlID="createAppointment"
                        DisplayModalPopupID="ModalPopupExtender1" />
           <ajaxToolkit:ModalPopupExtender  BackgroundCssClass="modalBackground" ID="ModalPopupExtender1" runat="server" TargetControlID="createAppointment" PopupControlID="PNL" OkControlID="ButtonOk" EnableViewState="false" />


           </ItemTemplate>
       </asp:TemplateField>

              </Columns>

Now when I select an option in my dropdown,the popup disapears, and i want it to stand still, how can i make it stand stil?