<div id="div_anonymous_popup" style="text-align:center;background-color: White;display:none; padding:20px;">
// content here
</div>

in head section write the following script codes.Before that jquery & jquery UI script
declarations needed -

<script type="text/javascript">
var dlg_pop;
function ShowPopUpDialog() {
dlg_pop = $('#div_anonymous_popup').dialog({
autoOpen: false,
modal: true,
width: 'auto',
height: 'auto',
title: 'Welcome'
});

$('#div_anonymous_popup').dialog('open');
dlg_pop.parent().appendTo('form');
}
</script>

then in code behind in Page_Load method -

runjQuery("ShowPopUpDialog();");

other related methods -

private string getCode(string jsCodetoRun)
{
StringBuilder sb1 = new StringBuilder();
sb1.AppendLine("function pageLoad() {");
sb1.AppendLine(jsCodetoRun);
sb1.AppendLine(" };");
return sb1.ToString();

}
private void runjQuery(string jsCodeforRun)
{
ScriptManager requestSM = ScriptManager.GetCurrent(Page);
if (requestSM != null && requestSM.IsInAsyncPostBack)
{
ScriptManager.RegisterStartupScript(this, typeof(Page), Guid.NewGuid().ToString(), getCode(jsCodeforRun), true);
}
else
{
Page.ClientScript.RegisterStartupScript(typeof(Page), Guid.NewGuid().ToString(), getCode(jsCodeforRun), true);
}
}

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