Tinymce 5.0.12 Visual Studio 2017 - cannot get content()

What should be a simple integration of tinyMCE in a Visual Studio C# application: I cannot get the HTML text after the user edits the content. I've tried all the tips and tricks I can find online. I must be missing something?

FIrst using straight jQuery to read the data after #submit button, return the content that was pre-set from code-behind, NOT what the user just changed.

First in my @ Page is set

     EnableEventValidation="false" ValidateRequest="false"

<script src="Scripts/tinymce/tinymce.min.js"></script>
     <script src="Scripts/tinymce/jquery.tinymce.min.js"></script>
     <script type="text/javascript">
        tinymce.init({
          selector: 'div#<%= txtPDFHeaderMessage.ClientID %>',
          height: 250,
          menubar: true,
          plugins: [
                'advlist autolink lists link image charmap print preview anchor textcolor',
                'searchreplace visualblocks code fullscreen',
                'insertdatetime media table paste code help wordcount'
          ],
          toolbar: 'undo redo | formatselect | bold italic backcolor | alignleft aligncenter alignright alignjustify | bullist numlist outdent indent | removeformat | help',
          content_css: [
            '//fonts.googleapis.com/css?family=Lato:300,300i,400,400i',
            '//www.tiny.cloud/css/codepen.min.css'
          ]
        });

$(document).ready(function() {
            //alert('ready');

            $('#btnSave').click(function () {
                var thisPDFHeaderMessage;
                thisPDFHeaderMessage= $('#<%= txtPDFHeaderMessage.ClientID %>').val();    // This return original content prior to editing.

               thisPDFHeaderMessage = tinyMCE.activeEditor.getContent();
               // active Editor.getContent() works...but I will have two text areas in my final solution.

               thisPDFHeaderMessage = tinymce.get($('#<%= txtPDFHeaderMessage.ClientID %>')).getContent();
               // this crashes...0x800a138f - JavaScript runtime error: Unable to get property 'getContent' of undefined or null reference

                alert('HERE ' + thisPDFHeaderMessage);
            });
        });
</script>

<label for="txtPDFHeaderMessage" style="margin-top:10px;"">Quote Intro Message</label>
    <div id="txtPDFHeaderMessage" class="mceEditor" runat="server"></div>
    <br />
    <button class="btn btn-primary" id="btnSave" value='0'><i class="glyphicon glyphicon-film icon-white"></i> Save</button>

I initialize the value of the as follows:

protected void Page_Load(object sender, EventArgs e)
        {
            if (!IsPostBack)
            {
               txtPDFHeaderMessage.InnerHtml = "Hello Testing";
            }
        }

Not sure what to try next?