Hi Mike,
Welcome to Microsoft Q&A forum.
I understand you would like to use your iCloud Contacts as the default Address Book in Classic Outlook for Windows, but the iCloud Contacts option does not appear when you open the Address Book.
In Classic Outlook, only contact folders that are enabled as an “e-mail Address Book” will show up in the Address Book list. Microsoft documents this behavior and the setting you need to check: Create address book
1, Make sure the iCloud Contacts folder is enabled as an address book
- In Outlook, go to People (Contacts).
- Right click the iCloud Contacts folder, then select Properties.
- Select the Outlook Address Book tab.
- Turn on “Show this folder as an e mail Address Book”, then select OK.
- Close and reopen Outlook, then open Home, Address Book and check whether the iCloud Contacts folder now appears in the Address Book list.
2, Set iCloud Contacts as the default Address Book (once it appears)
- Open the Address Book (Home tab, Address Book).
- Select Tools, then Options.
- Under “Show this address list first”, choose the iCloud Contacts address list, then select OK.
Reference: Change the default Address Book - Microsoft Support
If you do not see the “Outlook Address Book” tab in the folder’s Properties, Outlook will not be able to use that folder as an address book. In that case, the supported workaround is to create a new contacts folder under Contacts, enable it as an address book using the same “Show this folder as an e mail Address Book” option, and then move or copy the contacts you need into that Outlook contacts folder.
I hope this clarifies how Classic Outlook decides which contact folders can be used as an Address Book and how to set the default once it is available.
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.