The option to enter the gift code might not be available in some countries. In case it doesn't work in your country, use VPN and select US ip.
Create an account with any way you please (I prefer with email).
They will redirect you to the payment page, DONโT enter your payment details or anything else, we want it for free. At the top right you will find an option to enter the gift code, enter one of the gift code and click on Apply.
[Login to see the link]
Now you will be redirected to the next page, and here click on the Activate button.
[Login to see the link]
VOILA you got 3 month subscription for free.
Note: this works also if you already have an account with them, you can just enter the coupon code and you will get the 3 months.