You have created a calendar in the Nextcloud and want to offer it for subscription.
First you have to publish it:
1. click on the share icon
2. click on the plus icon
3. click on "Copy public link
So you get a URL that starts with https://, which anyone can use to view the calendar in their browser.
If you want to create a URL that users* can easily import into their own calendars, please replace the "https://" in your URL with "webcal://".
See the video: