Selecting a Dropdown List item Closes the Popup Window

In the code below; the "AutoPostBack" is set to true. When I select an item from the dropdown list; the popup window gets close automatically but returns the selected value. If I set "AutoPostBack" to false then it won't close popup but it will not return selected value.

What do I need to do so the popup won't close until I click on "Cancel"  or "Search" button?

<cc1:modalpopupextender id="ModalPopupExtender1" runat="server"
                OnOkScript="onModalOk();"
	            cancelcontrolid="btnCancel"
                okcontrolid="BtnSubmit"
	            targetcontrolid="lnkBtn_AddFromdatabase"
                popupcontrolid="panel1"
	            popupdraghandlecontrolid="PopupHeader" drag="true"
	            backgroundcssclass="ModalPopupBG"></cc1:modalpopupextender><asp:panel id="Panel1" style="display: none" runat="server"><div class="HellowWorldPopup"><div class="PopupHeader" id="PopupHeader">Database Search</div><div></div><br /><br /><div class="popup_Body" style="border:black 2px"><p>&emsp;&emsp;&emsp;Name:<asp:textbox id="txtName" runat="server"></asp:textbox></p><p>&emsp;&emsp;&emsp;<asp:Label ID="lblCompany" runat="server" Text="Company:"></asp:Label><asp:DropDownList ID="ddl_Company" runat="server" AutoPostBack="true" OnSelectedIndexChanged="OnSelectedIndexChanged_ddlCompany"></asp:DropDownList></p><p>&emsp;&emsp;&emsp;<asp:Label ID="lblID" runat="server" Text="ID:"></asp:Label><asp:DropDownList ID="ddl_Empployee" runat="server" AutoPostBack="true" OnSelectedIndexChanged="OnSelectedIndexChanged_ddlEmployee"></asp:DropDownList></p></div><br /><br /><div class="Controls" style="text-align:center"><input id="BtnSubmit" type="button" value="Search"/><input id="btnCancel" type="button" value="Cancel" /></div></div></asp:panel>

<style type="text/css">
    .ModalPopupBG
{
    background-color: #666699;
    filter: alpha(opacity=50);
    opacity: 0.7;
}

.AppPopup
{
    min-width:400px;
    min-height:275px;
    background-color:lightsteelblue;
    border-width: 3px;
    border-style: solid;
    border-color: white;
}
.popupBody
{
    min-width:400px;
    min-height:275px;
    background-color:lightsteelblue;
    border-width: 3px;
    border-style: solid;
    border-color: white;
}
.PopupHeader
{
    background:green;
    background-color:navy;
    color:yellow;
}
</style>

        protected void OnSelectedIndexChanged_ddlCompany(object sender, EventArgs e)
        {
            string sSelectedValue = "Selected Item: " + ddl_Company.SelectedItem.Text;
            ScriptManager.RegisterClientScriptBlock((sender as Control), this.GetType(), "alert", "alert('" + sSelectedValue + "');", true);
        }