how to create modal which will load video when user enters url from other website

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jQuery

bootstrap

i have tried with bellow code which directly displays video frame inside modal box

<div class="links" id="link6" data-toggle="modal" data-target="#myModalvid" title="Only Show Video Posts"></div> 
                             <div class="modal fade" id="myModalvid" role="dialog">
                                <div class="modal-dialog modal-sm" >
                                  <div class="modal-content" id="Div20">
                                    <div class="modal-header">
                                      <button type="button" class="close" data-dismiss="modal">&times;</button>
                                      <h4 class="modal-title">Add Video</h4>
                                      <asp:TextBox class="form-control" ID="txtvid" runat="server" placeholder="Enter Video URL"></asp:TextBox>
                                     </div>
                                    
                                    <div class="modal-body">
                                        <iframe width="550" height="350" src="//www.youtube.com/embed/mwuPTI8AT7M?rel=0" frameborder="0"></iframe>
                                    </div>
                                    <div class="modal-footer">
                                      <button type="button" class="btn btn-primary" >Post</button>
                                      <button type="button" class="btn btn-default" data-dismiss="modal">Close</button>                                      
                                    </div>
                                  </div>
                                </div>
                              </div>   

i not getting any idea how to implement it , when user enters Video URL in Textbox it will automatically display video in i-frame.