Before you can use an API, you have to create your application under "My Apps". Each Application can then be subscribed to APIs.
STEP 1: LOGIN
use this link to login to the developer portal
STEP 2: CREATE AN APPLICATION
use this link to create an application. You will need the generated client ID (API Key) and client secret (API secret) in a later step for creating your sandbox access token.
STEP 3: CREATE A TOKEN AND USE THE XS2A SANDBOX API
now you can already create a token for you app following Sandbox oauth flow and use the XS2A Sandbox API.