I haven't been to any BCG women's events, but can speak for my experience with McKinsey's and will assume they are fairly similar.
If this is a by-application-only event, the relevant recruiters will already have seen and screened your CV. You wouldn't necessarily need a "referral" per se, but having a recruiter chat to you and encourage you to apply (I know this happens at McKinsey) will at least give you a high chance of getting through the CV screening stage and straight to the tests.
In order to do this, I would just find some topics of conversation to chat about - it doesn't have to be BCG or your current job, but can be on other topics e.g. BCG's initiatives to encourage women to apply, some of their other current projects at BCG, etc. Get them talking to you and they'll ask you questions about yourself as well.
Importantly, keep in mind that the referral only helps get your CV through the first stage. From there, you are on a level playing field with everyone else who has passed that stage.
Best of luck!