Not working: redeem Apple Music code by registering in Beats by Dre

One week ago I have bought a new Beats X, here in Spain. They included a promo for three free months of Apple Music. "To redeem your offer, visit beatsbydre.com/register and use the code below", said the box. Well, the answer that i get from the web site is that they are having technical problems. When i register my Beats leaving blank the space for the promo code the registration does work. I have consulted with the technical support department, spoke to about 5 different agents, and they keep saying that the codes are been actualized because they belong before the purchase of Beats by Apple (three years ago) or that they are taking a week to solve technical problems at the Beats website. Does anybody hear knows how to issue a complaint and an alternative solution for this? Thanks.