Ok, I think I found a bug
I found out that I could not drag the behaviour in the "behaviour" field because the FSM in which I am trying to do that is defined via a template. If I was in a normal FSM, I could drag the behaviour without any problem.
PS: and yes, I eventually added "public" to the CarPlatformChange function, which should have been there in the first place.
note: added bug report in forum bug report topic.