Hello Markus,
cause the API is OAuth protected, did you all the necessary stuff for OAuth (e.g. the OAuth Client Config, OAuth App Config ...)? From your provided coding it seems, that it is not done, cause it is not used.
Detailed steps can be seen in post OAuth2 Authentication using HANA XS – XS OAuth client lib calling Google's API (2) which describes the setup for the usage of a Google API.
Regards,
Florian