if I recall corretly from my initilal application at BCG yu could just apply for a position (consulting in my case) and the set your office preferences in the end.
In my opinion the most important thing is that your CV data is in the BCG system - everything else can be adjusted later on.
I would recommend you double check with the person you got the referal from with the suggestion, taht you will apply for the position you want and then either don't set a location preference or for another office, if the first is not possible.
When I received my offer from BCG Germany I could chose which office I wanted to join. But that might be different in your specific case.
Hope that helps :)