My organ has a Midi-In, but I'm not sure if this signal would be forwarded to my computer. On the other hand I dont have and cant find a Midi-table of the organ, so not sure, whats in use. So I would rather not want to mess with it 
The moment the button is pressed, I just want to have one single signal (like a click) and none when it its released.
Here I found the Midi implementation chart, if thats any help (on page 27)
http://www.johannus.com/static/uploads/downloads/UM_Opus_EN.pdf