You cannot add internet calendars from the desktop application in Outlook 2016, Outlook 2019, or Outlook 365. For more information see this support link from Microsoft. Once added through the Web Application (OWA), you will be able to view the calendar on your Desktop Application as well.
ADDING A CALENDAR
1. Send a calendar subscription link to your email address using the Sync with Calendar or Mobile Calendar button on the Dashboard. Setting Up a Mobile Calendar Subscription
2. Open the email from Lightning Bolt Scheduler with the subject Calendar Subscription Notice.
3. Copy the .ics file link from under the Outlook section of the email.
4. Open the Outlook Web Application (OWA) and sign into your account.
5. Select the Calendar icon, to the right of the Mail icon in the bottom left hand corner of the screen.
6. Below New Calendar, select Discover calendars.
7. On the left column of the pop-up, under Import Calendars, select From Web
8. Paste the copied .ics link above into in the field Link to the calendar.
9. Change the field Calendar name to name the calendar.
10. Click Import.
11. Your Lightning Bolt calendar will now appear in the Other calendars section of the calendar list.
REMOVING A CALENDAR
1. Under the Other calendars section, select the three dot settings next to the calendar subscription you want to remove.
2. From the options that pop up to the right, select Remove.
3. Click Delete to remove the subscription from your Outlook calendar. You can always resubscribe through the procedures in the section Adding a Calendar.