First decide whether your app should write an event through an authorized calendar connection or offer an event for the person to add themselves.
Google Calendar
An app can use a Google Calendar connection to create events in the connected account. A workspace-owned connection writes to that workspace connection's calendar, not automatically to each visitor's calendar. To write to a customer's account, build and test the per-user connection and consent flow. See Choosing the Google Calendar account your app uses.
A calendar file
For an event the user adds themselves, ask the builder for an Add to calendar action that provides a standard .ics calendar file. Include the title, start and end, time zone and location. The calendar app decides how the file is imported; do not promise a silent write or a one-tap flow on every phone.
Test the complete flow on the target device and calendar app, including daylight-saving time and all-day events. A downloaded file does not prove the event was added. Provide clear import instructions for the calendar app you tested.
A read-only subscription calendar is another option when a schedule needs ongoing updates. Apple's calendar setup guide describes subscribing by calendar URL. A subscription and a one-time imported event have different update behavior; choose deliberately.
Builder changes use credits. Keep private event details out of publicly accessible calendar feeds.