Basically all you have to do is signup to their mailing list from that link and then in your email you get 20%:

Thanks and congratulations.

Print out this coupon and use in-store between 17th and 20th May for 20% off!

A Steaming Winter Offering For General Pants’ Blue-Blooded Family and Friends!

20% off your total purchases anytime between Thursday 17 May and Sunday 20 May.