On The Google Calendar Website

Since he originally asked how to click into the calendar and find the ID of a particular event, and since that's what I needed to know how to do too:

1. Click the event in your Google Calendar.

2. It will take you to a page with a URL such as https://calendar.google.com/calendar/render?pli=1#eventpage_6%7Ceid-NGh0Z3BtbTFobWFrNzQ0cjBrYmtkY29kYXIgZXVndTlrYW4xZGRpMXBtaTZzazNpYjWoNmdAZw-1-0-

3. Look for "eid" in the URL.

4. Select and copy the string between eid- and the next -. (In my example here, it is NGh0Z3BtbTFobWFrNzQ0cjBrYmtkY29kYXIgZXVndTlrYW4xZGRpMXBtaTZzazNpYjWoNmdAZw.)

5. Now you need to decode from Base64 format. You could paste that string into a tool such as https://www.base64decode.org/.

6. The result will be two strings separated by a space. The first string is your Event ID.

I understand that you want event-id for using https://www.google.com/calendar/event?eid={event-id}&ctz={timezone}. Is my understanding correct? If my understanding is correct, how about this modification?

Modification point :

• In order to retrieve event-id of https://www.google.com/calendar/event?eid={event-id}&ctz={timezone}, you can retrieve it by using Calendar.Events.list() of Calendar API.

Preparation to use this modified sample script :

When you use this modified script, please enable Calendar API at Advanced Google Services and API console.

Enable Calendar API v3 at Advanced Google Services

• On script editor
  • Resources -> Advanced Google Services
  • Turn on Calendar API v3

Enable Calendar API at API console

• On script editor
  • Resources -> Cloud Platform project
  • View API console
  • At Getting started, click Enable APIs and get credentials like keys.
  • At left side, click Library.
  • At Search for APIs & services, input "Calendar". And click Calendar API.
  • Click Enable button.
  • If API has already been enabled, please don't turn off.
  • When you run this script, if an error occurs, you might be required to wait few minutes until the API is enabled.

Modified script :

function export_gcal_to_gsheet(){
  var mycal = "info@neugier.schule";
  var cal = CalendarApp.getCalendarById(mycal);
  var today = new Date()
  var thisweek = new Date (today.getTime())
  var oneWeekfromNow = new Date(today.getTime() + (7*3600000*24));
  var events = cal.getEvents(thisweek, oneWeekfromNow, {search: 'stammtisch'});
  var sheet = SpreadsheetApp.getActiveSpreadsheet().getSheetByName("Termine");
  var header = [["Event Title", "Event Start", "Thema", "LinkZoom", "EventID"]]
  var range = sheet.getRange(1,1,1,5);
  range.setValues(header);

  // Added
  var items = Calendar.Events.list(mycal, {
    timeMin: thisweek.toISOString(),
    timeMax: oneWeekfromNow.toISOString(),
    q: 'stammtisch'
  }).items;

  for (var i=0;i<events.length;i++) {
    var row=i+2;
    var myformula_placeholder = '';
    var details=[[events[i].getTitle(), Utilities.formatDate(events[i].getStartTime(),'Europe/Berlin', 'dd.MM.yyyy,\' um\' HH:mm\'Uhr\''), events[i].getDescription(), events[i].getLocation(), items[i].htmlLink.split("=")[1]]]; // Modified
    var range=sheet.getRange(row,1,1,5);
    range.setValues(details);
  }
}

Note :

• This modified script retrieves htmlLink from Calendar.Events.list(). The link is https://www.google.com/calendar/event?eid={event-id}. Now I retrieved event-id from the link for matching your script using items[i].htmlLink.split("=")[1]. If you want the link, please modify it to items[i].htmlLink.

References :

• Advanced Google Services
• Calendar.Events.list()
• toISOString()

If I misunderstand your question, I'm sorry.