Bootstrap 4.3 - use code from below snippet - more herehere
$('#exampleModal').on('show.bs.modal', function (event) { let bookId = $(event.relatedTarget).data('bookid') $(this).find('.modal-body input').val(bookId) })a.btn.btn-primary.open-AddBookDialog {color: white }<!-- Initialize Bootstrap 4 --> <link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.3.1/css/bootstrap.min.css" integrity="sha384-ggOyR0iXCbMQv3Xipma34MD+dH/1fQ784/j6cY/iJTQUOhcWr7x9JvoRxT2MZw1T" crossorigin="anonymous"> <script src="https://code.jquery.com/jquery-3.3.1.slim.min.js" integrity="sha384-q8i/X+965DzO0rT7abK41JStQIAqVgRVzpbzo5smXKp4YfRvH+8abtTE1Pi6jizo" crossorigin="anonymous"></script> <script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.14.7/umd/popper.min.js" integrity="sha384-UO2eT0CpHqdSJQ6hJty5KVphtPhzWj9WO1clHTMGa3JDZwrnQq4sF86dIHNDz0W1" crossorigin="anonymous"></script> <script src="https://stackpath.bootstrapcdn.com/bootstrap/4.3.1/js/bootstrap.min.js" integrity="sha384-JjSmVgyd0p3pXB1rRibZUAYoIIy6OrQ6VrjIEaFf/nJGzIxFDsf4x0xIM+B07jRM" crossorigin="anonymous"></script> <!-- BUTTON --> <a class="btn btn-primary open-AddBookDialog" data-toggle="modal" data-target="#exampleModal" data-bookid="@book.Id" title="Add this item" >Open</a> <!-- MODAL --> <div class="modal fade" id="exampleModal" tabindex="-1" role="dialog"> <div class="modal-dialog" role="document"> <div class="modal-content"> <div class="modal-body"> <input type="hidden" name="bookId" id="bookId" value=""/> <input type="text" class="form-control" id="recipient-name"> </div> <div class="modal-footer"> <button type="button" class="btn btn-secondary" data-dismiss="modal">Close</button> </div> </div> </div> </div>