Bootstrap Minimal

HTML

<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.6/css/bootstrap.min.css">
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.6/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-hidden="true">×</button>
                <h4 class="modal-title">Subscribe our Newsletter</h4>
            </div>
            <div class="modal-body">
        <p>Subscribe to our mailing list to get the latest updates straight in your inbox.</p>
                <form>
                    <div class="form-group">
                        <input type="text" class="form-control" placeholder="Name">
                    </div>
                    <div class="form-group">
                        <input type="email" class="form-control" placeholder="Email Address">
                    </div>
                    <button type="submit" class="btn btn-primary">Subscribe</button>
                </form>
            </div>
        </div>
    </div>
</div>

JavaScript

$(window).load(function(){
        setTimeout(function(){ $('#myModal').modal({show:true}) }, 3000);
    });