To apply the Founding Membership you have to buy an Early Bird sim card and just enter the founding member discount code provided in your email. Follow the steps below to receive the discount.
Buy an Early Bird Sim on the website
Select the Data Allowance you would like and tap Add To Basket
Scroll down and tap Proceed To Checkout (you will not pay yet)
This page will require you to fill in your information. Make sure to enter the email which you subscribed to for the Founding Member Discount
Once the details are filled, scroll to the top and tap Have A Coupon? and enter the Founding Member code
You've received the discount and can check by scrolling down and checking the total price of the product.