If you want to archive iCloud emails directly in Mail Archiver then you have to use an application specific password. After you have created such a password you can use it in Mail Archiver.

In the Setup click on the Plus icon to create a new account. Select iCloud in the window for creating an IMAP account:

imap account type icloud

Enter your iCloud email address and the app specific password. The normal password does NOT work!!!

Enter username and app specific password for iCloud

If you entered the information correctly then the account should be created fine:

Finished iCloud account

Then you can add the account to your plan.

Updated 11-May-2023 with new screenshots.