Pick a directory from a list or use the default directory for the account:

GET https://management.azure.com/subscriptions
    ?api-version=2017-08-01
Authorization: {{$aadToken}}

Explicit directory using tenant ID:

GET https://management.azure.com/subscriptions
    ?api-version=2017-08-01
Authorization: {{$aadToken 00000000-0000-0000-0000-000000000000}}

Explicit directory using domain name:

GET https://management.azure.com/subscriptions
    ?api-version=2017-08-01
Authorization: {{$aadToken contoso.com}}

Do not reuse older token -- force re-authentication for current directory (e.g. switch account):

GET https://management.azure.com/subscriptions
    ?api-version=2017-08-01
Authorization: {{$aadToken new contoso.com}}

REST Client uses an in-memory token cache that clears when Visual Studio Code is restarted. You can clear the token cache manually by using F1 > Rest Client: Clear Azure AD Token Cache._

new can be used with any other options, as long as it's specified first. Order is important for all options._

Specify an explicit Azure AD app (aka audience or resource):

GET https://fabrikam.com/api/foo
Authorization: {{$aadToken aud:000000000000-0000-0000-0000-00000000}}

Audience (aud) must be an allowed value by the target API you are calling. Usually, this is a URL ending in a slash (/), but may also be an app/client id or other value. Contact the app owner to determine valid options._

Audience (aud) can be used with any other options, as long as it's specified last. Order is important for all options._

Implicit cloud selection (via REST endpoint TLD):

GET https://management.microsoftazure.de/subscriptions
    ?api-version=2017-08-01
Authorization: {{$aadToken}}

Explicit cloud selection:

GET https://management.usgovcloudapi.net/subscriptions
    ?api-version=2017-08-01
Authorization: {{$aadToken us}}

Azure China does not work like the other clouds. Use of Azure China is allowed, but may fail._

Use of Azure AD is not limited to Azure APIs. Inclusion here is for demonstration purposes only._

Thanks @flanakin for implementing this feature and providing following samples.