All you need to do is enter promotional code BAMember70 during checkout on ba.com.
This promo code is valid per person, and it's not just limited to Europe, it can be used on any long-haul destination that British Airways serves. The promo code will deduct from the total price including, taxes and all fees. It is also valid for all booking classes, while $70 isn't much of Business Class at least it's a start. This promo is only eligible for Executive Members in the US, and for departures leaving the US.
To use the promotion code, Executive Club Members must make a booking before 23:59 GMT April 23, 2018.
If you are not an Executive Club Member, you can join for free here; you must be logged in to receive the discount.