function onOpen() {
  SpreadsheetApp.getUi() // Or DocumentApp or FormApp.
      .createMenu('Custom Menu')
      .addItem('Show alert', 'showAlert')
      .addToUi();
}

function showAlert() {
  var ui = SpreadsheetApp.getUi(); // Same variations.

  var result = ui.alert(
     'Please confirm',
     'Are you sure you want to continue?',
      ui.ButtonSet.YES_NO);

  // Process the user's response.
  if (result == ui.Button.YES) {
    // User clicked "Yes".
    ui.alert('Confirmation received.');
    function skickaemail() {
      var file = DriveApp.getFileById('1gVPWRMEmJ68CxY1PKvwS-oPXVbteu7bvOCnQlZHPYsg');
      MailApp.sendEmail('email@email.se', 'försättsblad avtal', 'se bifogat innehåll.', {
         name: 'Automatic Emailer Script',
         attachments: [file.getAs(MimeType.PDF)]
      });
      }
  } else {
    // User clicked "No" or X in the title bar.
    ui.alert('Permission denied.');
  }
}