Hi there. The reward is connected to the platform account that you completed and claimed the triumph on.
Go to the Bungie Store and sign in with your platform account. You may be prompted to also re-authorize your Bungie.net profile if it's been a while.
Go to the [url=https://bungiestore.com/bungie-rewards-hall-of-champions-t-shirt]Hall of Champions T-Shirt product page[/url] and you should be able to choose your size and add the shirt to your cart for checking out.
Typically when completing triumphs that unlock Bungie Store rewards, you will also get an email notification that you earned the reward and instructions on how to claim it.
If you encounter issues, contact [url=https://bungiestore.com/customer-service]Bungie Store Customer Support[/url] for help.