function onOpen() {
  // Add a custom menu to the spreadsheet.
  SpreadsheetApp.getUi() // Or DocumentApp or FormApp.
  .createMenu('Custom Menu')
  .addItem('myToDoPanelLabel', 'showTestPanel')
      .addToUi();
}

function insert(value) {
  SpreadsheetApp.getUi().alert("Message"); //with this line it only works in test sheet but not in the original sheet
  var thisID="1Mv-0YRSJJpwSaRh_xAeZDtVqL-P-e5nS80fUJIq_CJ4";
  var ss = SpreadsheetApp.openById(thisID).getSheetByName("Tab1");
  ss.getRange(1,2).setValue("New Txt");

}


function showTestPanel() {
var template = HtmlService.createTemplateFromFile('panel');
  var htmlOutput = template.evaluate()
  .setTitle('ToDoPanel');
  SpreadsheetApp.getUi().showSidebar(htmlOutput);
}

<script>
  window.mark = function() {
   google.script.run.insert();
  };
</script>

<button onclick='mark()'>click me</button>