Please refer this example.
Here I have used Ajax AnimationExtender to show the Modal Popup from Left top to the center.
What you can do is set the horizontal position as 0 and vertical as some 400 points.
Ref: Building Modal Popup using ASP.Net AJAX ModalPopupExtender Control
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>
<style type="text/css">
.modalBackground
{
background-color: Black;
filter: alpha(opacity=90);
opacity: 0.8;
}
.modalPopup
{
background-color: #FFFFFF;
border-width: 3px;
border-style: solid;
border-color: black;
padding-top: 10px;
padding-left: 10px;
width: 300px;
height: 140px;
}
</style>
</head>
<body style="height: 3000px">
<form id="form1" runat="server">
<asp:ScriptManager ID="ScriptManager1" runat="server">
</asp:ScriptManager>
<asp:Button ID="btnShow" runat="server" Text="Show Modal Popup" />
<!-- ModalPopupExtender -->
<cc1:ModalPopupExtender ID="mp1" runat="server" PopupControlID="Panel1" TargetControlID="btnShow"
CancelControlID="btnClose" BackgroundCssClass="modalBackground">
</cc1:ModalPopupExtender>
<asp:Panel ID="Panel1" runat="server" CssClass="modalPopup" align="center" Style="display: none">
This is an ASP.Net AJAX ModalPopupExtender Example<br />
<asp:Button ID="btnClose" runat="server" Text="Close" />
</asp:Panel>
<!-- ModalPopupExtender -->
<cc1:AnimationExtender ID="popupAnimation" runat="server" TargetControlID="btnShow">
<Animations>
<OnClick>
<Parallel AnimationTarget="Panel1"
Duration="0.3" Fps="25">
<FadeIn />
<Move Horizontal="500" Vertical="250"></Move>
</Parallel>
</OnClick>
</Animations>
</cc1:AnimationExtender>
</form>
</body>
</html>
Screenshot
