Question about accruing FF miles with National- all these years I've just earned rental credits, never FF miles.

I'd like to earn United miles for the ongoing promotion. Can I combine a coupon for $$ off with the United contract ID? I think the answer is yes, but I'd like to double check.

It's a different system than Avis. At Avis, the coupon code gives you the mileage, thereby preventing you from being able to also use a coupon to discount the price of the rental.