An external service is not available in the interface form unless you don't subscribe to it. A service may be 'open' and does not require authorization. In that case it is enough if you subscribe through the dedicated form to be able to use it. On the opposite, if the service requires authorization, you must wait until the service owner accepts your subscription before you can use the service.
The owner may set some restrictions on the access. You will receive an email notification that informs you if the request was accepted or denied.