What do you mean by "activate it automatically"? The line is not supposed to be there normally, you are supposed to add it with a list of airports you want to disable for some reason, but by default it shouldn't be there.
In addition to that, it's not really useful right now because, if the reason you need to disable airports, it's because there are two nearby airports that are in conflict, those cases are better handled using the airport_visibility command instead.