Unfortunately there is not an available SDK for the KR203. However, there is one for the KR403 if you are interested. You can find out Link-OS SDK here: Link-OS Multiplatform SDK | Zebra
Please note that neither the KR203 or KR403 are Link-OS printers, so Link-OS specific capabilities will not be usable.
Hi Hugo, The KR203 uses a non-standard print language called KPL. It also requires some careful managing of the printer connection. You are better off programming to the driver than directly trying to connect to that printer.