Create an alias email address

You can add up to three 'email aliases' - these are additional addresses (not accounts) which deliver into the same inbox as the main account (which may not be what you want).

You should be aware before you start that once you've created an alias you cannot turn that address into a full iCloud account or move it to another account.

More information on aliases here: https://help.apple.com/icloud/#mm6b1a490a

If your wife is using the same user account on your Mac as you, your only other option is to create a new iCloud account (you will need to sign out of your Apple ID and sign back in with a new one for here, create the account, and then log back in: then sign in with her iCloud in System Preferences>Mail, Contacts & Calendars).

In neither case will there be privacy between accounts (should this be a requirement), but the latter method keeps the inboxes separate. However you can handle this just as well by creating an alias on your account, then setting up a Rule to move messages to her alias to a separate folder.

If you actually do want complete separation with privacy you will need to create a separate user account on your Mac for your wife, and for her to open a new iCloud on that.

Do you each have a user account (administrator account or otherwise on your Mac). Then each of you can have your own Apple ID and, thus, email account. See this Apple document Set up users, guests, and groups on Mac

https://support.apple.com/guide/mac-help/set-up-other-users-on-your-mac-mtusr001/mac Roger mentioned this approach above. I'd use before I'd use the other possible approaches.