You'd have to subclass (or copy paste & modify) one or more of the existing Crowd spring integration classes to do a search to find users by email if no user was found when searching by username.
Note that the integration (and the rest of Crowd) doesn't allow logging in with email by default because Crowd doesn't enforce unique email addresses across applications (which would be tricky to do because an application can have multiple directories, some of which may contain the same user with the same email under different usernames - currently that's legal but it would be ambiguous if we tried to enforce email uniqueness).
For JSM June Challenge #2, share how your non-technical teams like HR, legal, marketing, finance, and beyond started using Jira Service Management! Tell us: Did they ask to start using it or...
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