Wordpress Change Functionality Of TinyMCE Button U (underline)

does anybody know how to change functionality of button in wordpress content textarea? There is a 'u' button (underline) which makes text ></textarea> </form> <button id='get'>Test</button> <div id='previewHTML'></div> <div id='previewFormat'></div>

JS

var textArea_id = "#instance1";

tinymce.init({
   selector: textArea_id,
   toolbar: "underline",
   formats : {
         underline : {inline : 'u', exact : true},
   }
});


var button = document.getElementById('get');

button.onclick = function(){
   var contentHTML = tinymce.activeEditor.getContent({format: 'html'});
   document.getElementById('previewHTML').innerText = contentHTML;
   document.getElementById('previewFormat').innerHTML = contentHTML;
}

See this DEMO