These simple steps show how to add an iCloud calendar on Mac
Install OneCalendar
Download OneCalendar from the store and install it on your device.
Select Accounts overview
Select the OneCalendar menu on the top left, and select the option "Accounts".
Select Add account
Click on the "+" sign to add a new account.
Select Account type
You have 2 options for configuring iCloud calendar account.
Option 1 "iCloud on device": this is the easiest option to configure (it doesn't need a password) but is doesn't allow you to assign colours to individual appointments, and it's not possible to invite people for appointments.
Option 2 "iCloud": for this option you would need to setup an application specific password, it will be shown in the following steps.
Option 2: Enter credentials
Setup 2 factor authentication
All third-pary applications (like OneCalendar) require an app-specific password for connection to iCloud due to Apple security measures. In order to do this it's first required to setup 2 factor authentication. You only need to set this up once if you haven't done so already. This can be done on you iPhone/iPad or on your Mac.
For detailed instructions see Apple Support.
Create app specific password
For detailed instructions see Apple Support.