Usually the iOS standard way works fine with Zendesk WeekCal as well. Please try that first. If you have problems, try the following method instead.
On the Kalenders page, select a calendar you like.
Click Selecteren.
If you are not logged in to WebCal.Guru, you will not be able to modify subscription settings and you will have significantly smaller selection of calendars. In this case, jump directly to Step 4.
Adjust the subscription settings if needed and click Aan Mijn kalenders toevoegen.
You will be taken to Mijn kalenders page. On the calendar you just added, click next to Downloaden and select Adres naar klembord kopiëren.
On the home screen of the device, tap Settings.
Turn the device to portrait orientation, scroll down to Calendar and tap it.
Tap Accounts.
Tap Add Account.
Tap Other.
Tap Add Subscribed Calendar.
Tap the empty input field.
Tap Paste.
Tap Next.
Tap Save.
Updated 8 Feb 2023