Cashback is paid to your nominated credit card so make sure you put one on your profile or you can't get the cashback.
Not sure how many times you can use this promo. T&cs here: https://www.booking.com/content/referral-terms.en-gb.html?ai…
Also check the Spend $200USD get $40USD to see if it's better for you.
Happy travels!
Damn, I just booked yesterday.