Contoso is your verified custom domain, then the upn of the user1 will be username at domain.
<h3>Definition of a custom domain</h3>
A custom domain is known to be seen as a special form of branded name that tells only about a website. e.g. Marty custom domain is known to be Marty com.
Therefore, Contoso is your verified custom domain, then the upn of the user1 will be username at domain.
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