There is no official documentation for the current version, but Google had some documentation in the past. You can see it using web.archive wayback machine (it still works!): Share your events with an individual, a group, or the whole world.
https://calendar.google.com/calendar/render
required: yes
format: string/eval
possible values: TEMPLATE
example: action=TEMPLATE
description: A default required parameter.
required: yes
format: text
example: text=Birthday
description: event title.
required: yes
format: YYYYMMDDTHHmmSSZ/YYYYMMDDTHHmmSSZ
example: dates=20201231T193000Z/20201231T223000Z
description: gives the start and end dates and times (in Greenwich Mean Time) for the event. Dates must have both start and end time or it won't work. The start and end date can be the same (if appropriate). Special cases:
- to use the user's timezone:
20201231T193000/20201231T223000(don't specify a timezone); - to use UTC timezone, convert datetime to UTC, then use
Zsuffix:20201231T193000Z/20201231T223000Z; - for all-day events use
20201231/20210101. You must use the following date as the end date for a one day all day event, or +1 day to whatever you want the end date to be.
required: no
format: timezone name
example: ctz=America/New_York
description: custom timezone.
required: no
format: text
example: details=With clowns and stuff
description: description of your event.
required: no
format: text
example: location=North Pole
description: set location of the event. Make sure it's an address google maps can read easily.
required: no
possible values: AVAILABLE, BUSY, BLOCKING
format: string
example: crm=AVAILABLE
description: if Free, Busy, or Out of Office respectively.
required: no
possible values: true, false
format: string
example: trp=false
description: Show event as busy (true) or available (false).
Stands for RFC 5545 transparency.
It's ignored for all day events, please refer to crm instead.
required: no
format: key-value
example: sprop=website:www.santa.org&sprop=name:Sata-online
description: identify the website or event source
required: no
format: text (comma-separated emails)
example: [email protected],[email protected]
description: a list of guests (comma-separated).
required: no
format: text (email)
example: [email protected]
description: add an event to a shared calendar rather than a user's default.
required: no
format: text (RFC-5545 specs)
example: recur=RRULE:FREQ=DAILY
description: set recurring events.