In your browser, navigate to the Nextcloud Calendar app. Under "Settings & import", copy the "iOS/macOS CalDAV Address" into your clipboard.
Launch the Windows 10 Calendar app. Then, click the settings icon (gear icon) and select "Manage accounts".
Click "Add account" and choose "iCloud".
Enter an email, username and password. None of this information has to be valid-it will all be changed in the upcoming steps.
Click "Done". A message should appear indicating the settings were saved successfully.
In the "Manage Accounts" menu, click on the iCloud account created in previous steps, and select "Change settings". Then, click on "Change mailbox sync settings".
Scroll to the bottom of the dialog box, select "Advanced mailbox settings". Paste your CalDAV URL in the field labelled "Calendar server (CalDAV)".
After following all these steps, your Nextcloud calendar should synchronize. If not, check your username and password. Otherwise, repeat these steps.
NOTE: You will not be able to synchronize your calendar if you have two-factor authentication enabled. Follow the steps below to get an app password that can be used with the Calendar client app:
Log into Nextcloud. Click on your user icon, then click on "settings".
Special thanks to this Reddit user for their post: https://www.reddit.com/r/Nextcloud/comments/5rcypb/using_the_windows_10_calendar_application_with/