As of now it is not possible to subscribe to Calendar URL from Windows 10 Calendar app directly.
So you have to add iStudiez Pro Calendar URL to your Outlook or Google (or any other) account first.
After that - add that (Outlook/Google/etc.) account to Windows 10 Calendar app.
Sometimes adding Google account to Windows 10 Calendar app, some calendars under the "other calendars" category on Google Calendars (where iStudiez Pro Calendar URL gets) may not be syncing and showing up on Windows 10 Calendar app.
To solve this issue, go to https://calendar.google.com/calendar/syncselect and select the calendars to sync.