Create Event on Teamup Calendar permissions error

I am attempting to create an event in my Teamup calendar from javascript.

I send:

{ 
  method: 'POST',
  contentType: 'application/json',
  headers: 
  {  
    Accept: 'application/json',
    'Teamup-Token': <api_token>,
    Authorization: 'Bearer <auth_code>' 
  },
  payload: '{"subcalendar_ids":[0],"start_dt":"2024-08-24T14:15:22Z","end_dt":"2024-08-24T14:15:22Z","all_day":false,"tz":"Europe/London","remote_id":"string","title":"Test Title","notes":"Test Desc\n","location":"Home, EX6 6LQ","who":"Test Organiser","signup_enabled":false,"comments_enabled":false}',
  muteHttpExceptions: true 
}

To: https://api.teamup.com/<mycalendarurl>/events

But get:

{"error":{"id":"no_permission","title":"No permission","message":"You do not have the required permissions to do this. This may be due to a change in permissions of your link since you loaded the calendar. Please refresh the calendar."}}

I have requested the <auth_code> authentication token immediately before posting this request.
I haven’t changed permissions, and dont understand what “Please refresh the calendar” means (have quit from my browser, gone back to my calendar, refreshed that webpage etc…)

I am using <mycalendarurl> generated specifically for api calls with modify permissions in the Sharing section of the Teamup dashboard.
I generated <api_token> from the Request Teamup API Key page on the Teamup website

I can find no documentation on the required subcalendar_ids key, and have used 0 as per the example code hoping that indicates the first calendar

This is very close to working because it complains about the dates ‘does not match any accepted ISO8601 date format.’ if I include the 000 millisecond part from toISOString() as opposed to the permissions issue.