I am trying to create/delete events using smallest_created.ics and smallest_cancelled.ics.
smallest_created.ics content:
BEGIN:VCALENDAR
VERSION:2.0
PRODID:-//TEST cie//test//EN
METHOD:REQUEST
BEGIN:VEVENT
UID:1f22a659-bea8-4297-8f83-7ccf802da82f
STATUS:CONFIRMED
SEQUENCE:0
ORGANIZER;CN=??:MAILTO:ghost@test.com
ATTENDEE;CN=test abc test:MAILTO:abcdef@test.com
SUMMARY:Test smallest
DTSTART:20210122T230000Z
DTEND:20210123T000000Z
DTSTAMP:20210121T141557Z
END:VEVENT
END:VCALENDAR
smallest_cancelled.ics content:
BEGIN:VCALENDAR
VERSION:2.0
PRODID:-//TEST cie//test//EN
METHOD:CANCEL
BEGIN:VEVENT
UID:1f22a659-bea8-4297-8f83-7ccf802da82f
STATUS:CANCELLED
SEQUENCE:1
ORGANIZER;CN=??:MAILTO:ghost@test.com
ATTENDEE;CN=test abc test:MAILTO:abcdef@test.com
SUMMARY:Test smallest
DTSTART:20210122T230000Z
DTEND:20210123T000000Z
DTSTAMP:20210121T141614Z
END:VEVENT
END:VCALENDAR
I send an email containing both files to an Office365 account (opens to https://outlook.office365.com/) and to an outlook account (opens to https://outlook.live.com/).
For both accounts, I can add the event to the calendar through the website and through the outlook desktop app.
However, it fails to cancel the event if I do "Add to calendar" in the browser.
chrome debugger
If I open the outlook desktop app and I do "Open" on smallest_cancelled.ics, I get a prompt saying "Add this Internet Calendar to Outlook?". When I press "Yes", a new window opens with the event and the option to remove it from my calendar
cancelation with METHOD:CANCEL in outlook desktop app gif
At this point, I can close the window and I'll have the event appear the same in my calendar.
Based on the RFC 2446 section 3.2, which is referenced by the RFC 5545 section 3.7.2, METHOD:CANCEL is the right way to indicate that an event is canceled.
As posted in this StackOverflow thread, changing the content of smallest_cancelled.ics to METHOD:REQUEST will make it work if I try to cancel the event through the website. However, if I open the outlook desktop app and I do "Open" on smallest_cancelled.ics, I get a prompt saying "Add this Internet Calendar to Outlook?". When I press "Yes", a new window opens with the event and I no longer have the option to remove it from my calendar
cancelation with METHOD:REQUEST in outlook desktop app gif
I believe the behavior of the outlook desktop app with METHOD:CANCEL to be the right one.
How can I cancel the event through both the website and the desktop app without sending 2 cancelation ics files?
question from:
https://stackoverflow.com/questions/65831294/cant-cancel-an-event-with-an-ics-from-the-browser-but-in-works-in-the-desktop 
