Request for authenticating an API user.
Use your assigned client_id and client_secret in base64 in the Authorization header.
You can find this information in your Resources section on PayTomorrow as API Signature, which is the client_id and client_secret already in base64.
Authorization: Basic YmxhYmxhYmxhOmJsYWJsYWJsYQ==
Returns a json with the token and the refresh_token.
Please note, scope should always be openid.