<!-- BEGIN COPY / PASTE -->
        <form id="myForm" class="needs-validation" novalidate>
            <div class="input-group mb-3">
                <input type="text" class="form-control" id="username" placeholder="Username" required>
                <div class="invalid-feedback">
                    Please enter a valid username.
                </div>
            </div>
            <button class="btn btn-primary" type="submit">Submit</button>
        </form>

        <script>
            (function() {
                'use strict';
                var form = document.getElementById('myForm');
                form.addEventListener('submit', function(event) {
                    if (!form.checkValidity()) {
                        event.preventDefault();
                        event.stopPropagation();
                    }
                    form.classList.add('was-validated');
                }, false);
            })();
        </script>
        <!-- END COPY / PASTE -->