Hi Jimmy_luu,
The details for the promotion you appear to be describing can be found at Get a pair of Beats Flex earphones on us, with your Apple Music student subscription. Based on those terms, if you have not previously subscribed to Apple Music or redeemed any other promotional offer, you should expect an email within 30 days of making your first payment for the service, and the instructions in that email must be completed within 90 days.
If the terms from that page have been met or you have additional questions, use Apple Support to connect with AppleCare directly to investigate further.
Regards.