How do I redeem my 3 months of free Apple Music with the code I got from the purchase of my Beats?

So I bought some BeatsX a while ago and never redeemed the code for 3 months of free Apple Music you get in the box. On the card that has the code, it says “To redeem your offer, visit beatsbydre.com/register and use the code below”. However, when you go to that website, it only asks for your Beats’ serial number, your name, and your email in order to register your beats. There is no place to put a code to redeem 3 months of Apple Music. I googled this problem and noticed that I was not the only one experiencing it. I also never found an answer on how to solve it. If someone could help, it would be much appreciated. Thanks