The Kolay Gelsin API is really the Sendeo service
This is the first thing that confuses people looking for integration docs: the Kolay Gelsin web service runs on Sendeo infrastructure. The live endpoint is api.sendeo.com.tr, while the integration/test environment is served from apiintg.kolaygelsin.com. Seeing two brand names does not mean you are in the wrong place — they are two names for the same service.
Authentication happens through a LoginAES call that expects two values: a customer code and a password. On the Stokzone side you only enter those two; the password is stored encrypted and never shown in the interface again. If you want to work against the test environment you just tick the box — no separate URL to type in.