There is a way to accomplish this. Book the round trip to take advantage of the coupon/promo. Then, immediately call WN CS and ask the rep to cancel just the return segment. They are going to ask you if you want the fare for this segment held as TTFs. Say no and ask for a full refund to your credit card instead. You are entitled to the refund option since you are canceling within 24 hours of booking. This has worked for me for the last few coupons/promos requiring R/T purchases. YMMV.