From https://university.athento.com go to See Courses (the blue button).
Click on the course you want to take:
Now go to "Start learning" (it might say "Register" or "Enroll" in some courses). You can also use the blue button at the bottom of the page.
Both buttons will take you to the checkout page. If you have a coupon, you can use it from there:
Provide the coupon and your email. Then, click on Buy Now.