In the Prestashop backend I activated the webservice and I created an authentication key. Testing in browser works well but now I want to access the webservice via JavaScrip
Finally, I figured out how to send the ws_key:
$http({ method: 'get', url: ws_url + 'customers/?schema=blank', params: { ws_key: ws_key });
You have to pass the ws_key param with your key to access the PrestaShop webservice.