1. create modal structure

                <div id="myModal" class="modal fade" tabindex="-1">
                        <div class="modal-dialog">
                                <div class="modal-content">
                                
                                </div>
                        </div>
                </div>

                

2. add modal header

                <div class="modal-header">
                    <h5 class="modal-title">Modal Title</h5>
                    <button type="button" class="close" data-dismiss="modal">&times;</button>
                </div>
                

3. add modal body

                <div class="modal-body">
                    <p>This is a simple Bootstrap modal. Click the "Cancel button", "cross icon" or "dark gray area" to close or hide the modal.</p>
                </div>
                

4. add modal footer

                <div class="modal-footer">
                    <button type="button" class="btn btn-secondary" data-dismiss="modal">Cancel</button>
                    <button type="button" class="btn btn-primary">Save</button>
                </div>
                

Complete code

                         <!-- Modal HTML -->
                        <div id="myModal" class="modal fade" tabindex="-1">
                                <div class="modal-dialog">
                                        <div class="modal-content">

                                                <div class="modal-header">
                                                        <h5 class="modal-title">Modal Title</h5>
                                                        <button type="button" class="close" data-dismiss="modal">&times;</button>
                                                </div>

                                                <div class="modal-body content_panel">
                                                        <p>This is a simple Bootstrap modal. Click the "Cancel button", "cross icon" or "dark gray area" to close or hide the modal.</p>
                                                </div>

                                                <div class="modal-footer">
                                                        <button type="button" class="btn btn-secondary" data-dismiss="modal">Cancel</button>
                                                        <button type="button" class="btn btn-primary">Save</button>
                                                </div>
                                        </div>
                                </div>
                        </div>

                        

5. show the modal

1. add button

                        <button type="button" class="btn btn-lg btn-primary">Show Modal</button>
                        

2. add click event

                        <script>
                                $(document).ready(function(){
                                        $(".btn").click(function(){
                                                $("#myModal").modal('show');
                                        });
                                });
                         </script>
                        

load the content using ajax

                      $(".content_panel").load("demo_test.txt");
                      

                      <script>
                                $(document).ready(function(){
                                        $(".btn").click(function(){
                                                $("#myModal").modal('show');
                                                $(".content_panel").load("demo_test.txt");
                                        });
                                });
                         </script>