Go to
https://morphic.com/pricing
Make sure monthly billing is selected.
Choose the 'Pro' plan and click 'choose plan.'
On the checkout page, click 'add promotion code' (bottom left).
Enter the code 'CONTRA' to redeem your free Pro plan for the duration of the challenge.
When I click "choose plan" it simply takes me to a page inside the platform. I created an account and tried it again, but don't get "add promotion code" on the checkout page.