Ah yes, forgot about that!
The sendHttpPostRequest method which includes the headers is available from OH 3.0
Since I am running 2.5 is used a work around.
You need to put the following jar in your addons folder:
https://github.com/jimtng/binaries/blob/master/org.openhab.core.model.script-2.5.0.jar
Thanks to @jimtng in this topic: https://community.openhab.org/t/solved-how-to-use-sendhttppostrequest-with-http-headers/104396/4