Using iCloud email when the Apple ID isn't the same

My spouse's Apple ID is her gmail address. Recently she added an iCloud email address within iCloud online, and it works fine. She wants to use that address on the mail app on her MacBook Pro (as well as on her other devices), but when she turns iCloud email on within settings, it asks her to create a new email address (i.e., it doesn't recognize that she already has one). If she enters the new iCloud email address, it says it is already taken. How does she use the new iCloud email address she just created (within her existing iCloud account under the Apple ID that is the gmail address) on her various devices?