<button style="margin:0px" type="button" class="btn btn-success" data-toggle="modal" data-target="#test" onclick="document.getElementById('SelectedDateId').value='@ModalString';document.getElementById('exampleModalLongTitle').innerHTML='@ModalString';" >
                                                                        Reservieren
                                                                    </button>

                                                                    <!-- Modal -->

                                                                    <div class="modal fade" id="test" tabindex="-1" role="dialog" aria-labelledby="exampleModalCenterTitle" aria-hidden="true">
                                                                        <div class="modal-dialog modal-dialog-centered" role="document">
                                                                            @using (Html.BeginForm())
                                                                            {
                                                                                <div class="modal-content">
                                                                                    <div class="modal-header">

                                                                                        <h5 class="modal-title" id="exampleModalLongTitle"></h5>

                                                                                        <button type="button" class="close" data-dismiss="modal" aria-label="Close">
                                                                                            <span aria-hidden="true">&times;</span>
                                                                                        </button>
                                                                                    </div>

                                                                                    <div class="modal-footer">
                                                                                        <input type="hidden" name="SelectedDateId" id="SelectedDateId" />
                                                                                        @Html.Label("Comments")
                                                                                        @Html.TextBoxFor(model => model.SelectedText)


                                                                                        <button type="button" class="btn btn-secondary" data-dismiss="modal">Abbrechen</button>
                                                                                        <input type="submit" value="Reservieren" class="btn btn-primary" />
                                                                                    </div>
                                                                                </div>

                                                                            }
                                                                        </div>
                                                                    </div>