How do I code a HTML event email that will be automatically added to users calendar, as per emails you receive about flights, etc?

Is there a standard for coding HTML emails for events that will automatically have the event be added to recipients calendars, in a similar way to how emails you receive about flights now get automatically added to your google calendar?

I can only find details about creating and attaching calendar events form say gmail, but can't see if there is any way to just include code within a HTML email that will make the email be automatically set as a calendar event.

Any thoughts would be great!