title: How to specify audience for an OAuth2 access token?
body: <p>I am confused that there seems to be no standard way to specify the audience for an access token when sending an authorization request to an authorization server.</p>
  
  <p>OAuth2 specifies access tokens as opaque strings; there is only one mention of 'audience' in the spec, to say that access tokens could be 'audience-restricted'.
  Many recent authorization server implementations seem to produce JWT access tokens, and JWT specifies the audience (aud) claim.</p>
  
  <p>As far as I find:
  - Auth0 uses an 'audience' parameter
  - Connect2id uses a 'resource' parameter
  - Identity Server uses a fixed issuer-based value for 'aud' claim, and assumes that scopes are enough - however, this does not fit all use cases.
  - The excellent 'OAuth2 in Action' book shows an example with a resource server URI in the 'aud' claim, but doesn't say where it comes from.</p>
  
  <p>So, how to get an access token for a specific audience (resource server, API,...) in a standard way?</p>