Authenticating with username instead of e-mail? (installing self hosted)

I'm building an app that'll be used internally at my workplace. The users won't be using their email, phone number, etc. Rather, I'd like to provide them with a username and password so they can log in.
They won't be registering their own accounts so I can generate them on my end and just provide credentials as required.

Is there an efficient way to log in by providing a username field value instead of e-mail?
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