Hi wozniak,
Please refer below sample code.
HTML
<html xmlns="https://www.w3.org/1999/xhtml">
<head id="Head1" runat="server">
<meta charset="utf-8" />
<meta http-equiv="X-UA-Compatible" content="IE=edge" />
<meta name="viewport" content="width=device-width, initial-scale=1" />
<title>Bootstrap 101 Template</title>
<!-- Bootstrap -->
<link href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.2/css/bootstrap.min.css"
rel="stylesheet" />
<!-- HTML5 shim and Respond.js for IE8 support of HTML5 elements and media queries -->
<!-- WARNING: Respond.js doesn't work if you view the page via file:// -->
<!--[if lt IE 9]>
<script src="https://oss.maxcdn.com/html5shiv/3.7.2/html5shiv.min.js"></script>
<script src="https://oss.maxcdn.com/respond/1.4.2/respond.min.js"></script>
<![endif]-->
<script type="text/javascript">
function ShowPopup() {
$("#btnShowPopup").click();
}
</script>
</head>
<body>
<form id="form1" runat="server">
<div class="container">
<div class="row">
<button type="button" style="display: none;" id="btnShowPopup" class="btn btn-primary btn-lg"
data-toggle="modal" data-target="#myModal">
Launch demo modal
</button>
<!-- jQuery (necessary for Bootstrap's JavaScript plugins) -->
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.2/jquery.min.js"></script>
<!-- Include all compiled plugins (below), or include individual files as needed -->
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.2/js/bootstrap.min.js"></script>
<div class="modal fade" id="myModal">
<div class="modal-dialog">
<div class="modal-content">
<div class="modal-header">
<button type="button" class="close" data-dismiss="modal" aria-label="Close">
<span aria-hidden="true">×</span></button>
<h4 class="modal-title">
Registration done Successfully</h4>
</div>
<div class="modal-body">
<asp:Label ID="lblMessage" runat="server" />
</div>
<div class="modal-footer">
<button type="button" class="btn btn-default" data-dismiss="modal">
Close</button>
<button type="button" class="btn btn-primary">
Save changes</button>
</div>
</div>
<!-- /.modal-content -->
</div>
<!-- /.modal-dialog -->
</div>
<!-- /.modal -->
</div>
</div>
</form>
</body>
</html>
Code
C#
protected void Page_Load(object sender, EventArgs e)
{
ClientScript.RegisterStartupScript(this.GetType(), "alert", "ShowPopup();", true);
this.lblMessage.Text = "Your Registration is done successfully. Our team will contact you shotly";
}
VB.Net
Protected Sub Page_Load(sender As Object, e As EventArgs) Handles Me.Load
ClientScript.RegisterStartupScript(Me.GetType(), "alert", "ShowPopup();", True)
Me.lblMessage.Text = "Your Registration is done successfully. Our team will contact you shotly"
End Sub
Screenshot