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

        <script>
            // JavaScript for disabling form submissions if there are invalid fields
            (function () {
                'use strict';
                var forms = document.querySelectorAll('.needs-validation');
                Array.prototype.slice.call(forms).forEach(function (form) {
                    form.addEventListener('submit', function (event) {
                        if (!form.checkValidity()) {
                            event.preventDefault();
                            event.stopPropagation();
                        }
                        form.classList.add('was-validated');
                    }, false);
                });
            })();
        </script>
        <!-- END COPY / PASTE -->