For Google Workspace Add-ons, an Attachment class has been added to the Card Service that lets you add custom attachments to Calendar events | Release Notes | March 24, 2022 |  Apps Script  |  Google Developers

March 24, 2022

For Google Workspace Add-ons, an Attachment class has been added to the Card Service that lets you add custom attachments to Calendar events. You can also set an event trigger that fires when the user clicks on the add-on attachment provider in the Calendar dropdown menu. For more information, refer to EventAttachmentTrigger.

You can read more about attaching files to Google Calendar events in this related Google Workspace Updates post:

Google Workspace developers can now create Google Workspace add-ons that attach files to a Google Calendar event from any third-party service. This feature enables developers to create add-ons that support attachments from a wide range of sources beyond Google Drive, such as digital whiteboard, content creation, or file management tools.

The update includes the following additional guidance for developers:

You can now develop add-ons that support attaching files from sources other than Google Drive. To get started, learn more about extending Calendar with Google Workspace add-ons and adding attachments with a callback function.

Source: Release Notes  |  Apps Script  |  Google Developers