Welcome to the Community !
If you received the offer between October 25, 2017 and July 30, 2018, you can redeem the promo code for your first 6 months free of a Spotify Premium subscription until July 31, 2019, as long as you are a first time Spotify Premium subscriber.
You would have to redeem it on Spotify.com directly.
Note that you need to have your promo code, we cannot generate new ones on our side but we can re-send it to you if you do not have it anymore
Don't hesitate to contact us for more assistance with this