User-1330468790 posted
Hi jagjit saini,
You can directly refer to the documentation about how to call a modal via Javascript on Bootstrap website:
https://getbootstrap.com/docs/4.0/components/modal/#via-javascript
More details, you could refer to below codes:
aspx:
<head runat="server">
<title></title>
<!-- Latest compiled and minified CSS -->
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.4.1/css/bootstrap.min.css"/>
<!-- jQuery library -->
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.5.1/jquery.min.js"></script>
<!-- Latest compiled JavaScript -->
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.4.1/js/bootstrap.min.js"></script>
</head>
<body>
<form id="form1" runat="server">
<div>
<!-- Button trigger modal -->
<button type="button" class="btn btn-primary" data-toggle="modal" data-target="#exampleModal">
Launch demo modal
</button>
<!-- Modal -->
<div class="modal fade" id="exampleModal" tabindex="-1" role="dialog" aria-labelledby="exampleModalLabel" aria-hidden="true">
<div class="modal-dialog" role="document">
<div class="modal-content">
<div class="modal-header">
<h5 class="modal-title" id="exampleModalLabel">Modal title</h5>
<button type="button" class="close" data-dismiss="modal" aria-label="Close">
<span aria-hidden="true">×</span>
</button>
</div>
<div class="modal-body" id="modalContent">
...
</div>
<div class="modal-footer">
<button type="button" class="btn btn-secondary" data-dismiss="modal">Close</button>
<button type="button" class="btn btn-primary">Save changes</button>
</div>
</div>
</div>
</div>
</div>
<asp:Button ID="ShowModalBtn" runat="server" Text="Show Modal" OnClick="ShowModalBtn_Click" />
<script type="text/javascript">
function ShowPopup(message) {
$("#modalContent").html(message);
$('#exampleModal').modal('toggle');
};
</script>
</form>
</body>
Code behind:
protected void ShowModalBtn_Click(object sender, EventArgs e)
{
ClientScript.RegisterStartupScript(this.GetType(), "Popup", "ShowPopup('this is an error');", true);
}
Demo:

Hope helps.
Best regards,
Sean