User:BZPN/Email.js

mw.loader.using(['mediawiki.api', 'jquery'], function () {
  $(function () {
    if (mw.config.get('wgCanonicalSpecialPageName') !== 'EmailUser') return;

    var $copyField = $('label[for="ooui-php-6"]').closest('.mw-htmlform-field-HTMLCheckField');
    if (!$copyField.length) return;

    var $notifyCheckbox = $('<div class="mw-htmlform-field-HTMLCheckField">' +
      '<div class="oo-ui-fieldLayout-field">' +
        '<input type="checkbox" id="notifyOnTalk" name="notifyOnTalk" value="1"> ' +
        '<label for="notifyOnTalk">Notify user on talk page</label>' +
      '</div>' +
    '</div>');

    $copyField.after($notifyCheckbox);

    $('form.mw-htmlform').on('submit', function (e) {
      if ($('#notifyOnTalk').is(':checked')) {
        e.preventDefault();

        var targetUser = $('#mw-input-wpTarget').val();
        if (!targetUser) return this.submit();

        var talkPage = 'User talk:' + targetUser;
        var text = '\n\n== You\'ve got got mail! ==\n{{You\'ve got mail}}'+'~~'+'~~\n';

        new mw.Api().postWithToken('csrf', {
          action: 'edit',
          title: talkPage,
          appendtext: text,
          summary: 'Notification: You\'ve got mail!',
          format: 'json'
        }).always(() => $('form.mw-htmlform').off('submit').submit());
      }
    });
  });
});