thx it is now working!
My solution (for future readers):
item:
Switch Leinwand "Leinwand" (EG_Wohnzimmer) { alexa="Switchable", channel="http:url:leinwand:switch"}
thing:
Thing http:url:leinwand "Leinwand" [ baseURL="base-url", commandMethod="POST"] { Channels: Type switch : switch "Switch" [ commandExtension="%2$s", onValue="sub-url-on", offValue="sub-url-off"] }
*edit: sourcecode is so much more pretty now <3