As per the Azure API docs, the element Domain is optional here. Do we want to consider making the domain optional in our code too?
"Optional. Specifies the name of the domain used to authenticate an account. The value is a fully qualified DNS domain. If the domains name is not specified, Username must specify the user principal name (UPN) format (user@fully-qualified-DNS-domain) or the fully-qualified-DNS-domain\username format"