WhatsApp, Instagram, Facebook and Messenger open links in their own built-in browser. On iPhone, that browser cannot add a calendar file directly, so when you tap Apple or Other Calendar on an AddCal event page, AddCal shows an Add to your calendar sheet that explains what happens next.
Add the event from the app
The sheet says the app's built-in browser can't add events directly, and that iOS will ask you to subscribe to the event.
Tap Add to Apple Calendar (or Add to calendar if you chose Other Calendar).
When iOS asks, tap Subscribe. The event then appears in your calendar.
Add it as a single event instead
If you would rather add a one-off event than a subscription, open the link in your phone's normal browser:
Tap the menu in the top corner of the app's browser.
Choose Open in Safari or Open in external browser (the wording depends on the app).
Add the event from the page that opens.
For organisers
If you share event links in WhatsApp or social posts on the Business plan, you can use a Dynamic Link without a calendar service chosen. It opens a page where each person picks their own calendar. See Add-to-calendar links for the other ways to share an event.