Found it in the email. Weirdly, I couldn't find any mention of the access link anywhere on the website:
To activate the product(s), follow the steps listed below:
1. To access the GMAT Official AWA product, please use the link below.
https://gmat.adaptera.com/express/login.htm2. Click on the “Student Self-Registration” button for at the bottom of the page to access the platform
3. Fill in your name, email, and password
4. Enter your 15-digit Activation Code in the text box for the purchased product
5. Click Submit