function onOpen() {

  var ui = SpreadsheetApp.getUi();

  ui.createMenu('Custom Menu')

      .addItem('Open Custom Dialog', 'openCustomDialog')

      .addToUi();

}

​

function openCustomDialog() {

  var html = HtmlService.createHtmlOutputFromFile('Page')

      .setWidth(300)

      .setHeight(200);

  SpreadsheetApp.getUi().showModalDialog(html, 'Custom Dialog');

}

​

function processSelection(selection) {

  var sheet = SpreadsheetApp.getActiveSpreadsheet().getActiveSheet();

  sheet.getRange('A1').setValue(selection);

}