Get username email from ad c#

// Retrieve a user by id
var user  = await graphClient
    .Users["00000000-0000-0000-0000-000000000000"]
    .Request()
    .GetAsync();

// Retrieve a user by userPrincipalName
var user  = await graphClient
    .Users["[email protected]"]
    .Request()
    .GetAsync();