Hi and welcome,
the bad thing is, the API documentation is reaaaally outdated such 2010’thing. It will soon be updated (I can’t await it, too).
If you like, you could have a look at the current partner area and watch the ajax calls like (reverse engineering):
'https://partner.spreadshirt.com/api/v1/users/114919634/statistics/creditDetails?apiKey=XXX&limit=20&locale=de_DE&mediaType=json&offset=0&sig=XXX&time=1543263299423'
If you authenticate, the usual way (via api), before calling the link, you should be able to retrieve data.
I sadly can’t tell you, if this will be covered in the updates of the api documentation nor the behavior will be same, after releasing the new api officially.
Hope it will help you nevertheless
Best,
Thimo