We can write a function in Java script as
function PageLoad()
{
document.getElementById("btn").click();
}
and call it in Page Load as
<body onload="PageLoad()" >
and place a button control in page.
& for ModalPopUp,
<ajaxToolkit:ModalPopupExtender ID="ModalPopUp" runat="server" BackgroundCssClass="modalBackground"
cancelcontrolid="CancelButton" dropshadow="true" okcontrolid="OkButton"
popupcontrolid="Panel1" targetcontrolid="btn">
btn is ID of button control.
It will work.