Like any office, they prefer candidates who naturally "fit" there. I.e. are Swiss citizens, have worked in Switzerland, speak French (or German or Swiss-German), etc.
Secondarily, you'll at least want an EU citizenship, and have worked in Europe + speak a few languages.
Regardless, I highly recommend you network. Like anywhere, you can get in without a referral, but a referral greatly helps. Furthermore, given what appears to be your limited knowledge of the office itself, networking will make you far more prepared come interview day (in terms of what to expect, what to answer for "Why BCG Geneva", etc).