I watch a youtube video and the youtuber shills me some product along with a promo code. A week later I decide to buy the product, but I don't remember what vid

I watch a youtube video and the youtuber shills me some product along with a promo code. A week later I decide to buy the product, but I don't remember what video had the ad or what the promo code was so I just don't bother with it. How are you going to attribute this sale to the youtuber?