in src/Unosquare.PassCore.PasswordProvider/PasswordChangeProvider.cs [147:156]
private string FixUsernameWithDomain(string username)
{
if (_idType != IdentityType.UserPrincipalName) return username;
// Check for default domain: if none given, ensure EFLD can be used as an override.
var parts = username.Split(new[] { '@' }, StringSplitOptions.RemoveEmptyEntries);
var domain = parts.Length > 1 ? parts[1] : _options.DefaultDomain;
return string.IsNullOrWhiteSpace(domain) || parts.Length > 1 ? username : $"{username}@{domain}";
}