Even when you are registered on the race, you can still add your medal engraving or any other product from your Maxfun profile:
Sign up on your Maxfun profile HERE.
Once inside your profile, you have to click on the drop-down menu that appears on the right of your registration in "VIEW REGISTRATION"
Once this option is selected, scroll down until you find the box "ADD PRODUCT":
You will see now all products available for the race.
To buy them, you just need to click on buy, choose the payment method and accept.