How to create simple add to calendar text link?

This is not a WordPress question, since WP does not have calendars (besides the Calendar widget showing posts).
The answer would depend on what your calendar is based on.

I see. The calendar date would be based on a date entered via a custom date field in WP post.

A simple Google search showed me these (and more):
https://stackoverflow.com/questions/5831877/how-do-i-create-a-link-to-add-an-entry-to-a-calendar
https://www.labnol.org/apps/calendar.html

You can see that it’s not as simple as you think, and it depends on the calendar.