This promotion is valid until November 11th for hotel stays before March 29th, 2020. To use this promotion, you need to go to the deal’s landing page. At the bottom, click on “book”, and then select your hotel and travel dates. Once redirected to the hotel’s booking site, you should see a rate named POWER YOUR EXPERIENCE PROMO. Also, make sure to be logged in with your (free) NH Rewards account to get the 10% discount.
Here’s an example of the difference this promotion makes, based on the NH Buenos Aires 9 de Julio: