Ignacio,

many thanks for reply! Your hint got me on the right track. The complete URL (Learning Locker in my case) should be:

http://localhost:9001/?endpoint=https://my-lrs.com/data/xAPI&auth=Basic NGFhMWV.....&actor={"objectType":"Agent","mbox":"mailto:test@user.com","name":"Test User"}

The values for endpoint and auth are provided by the LRS.