Samara's design was chosen from the final two after a vote involving youth and those working with them.
Ms Birch said more than 400 votes were cast and Samara's design came out on top.
She was then teamed up with Hot Printz' senior designer Brett Sands and her design was put onto a T-shirt.
The 500 T-shirt run has already sold out.
"We normally sell 300 to 400, so that's a record number sold and we could have sold many more," she said.
Ms Birch said Samara's design was a "quite funky" cartoon style that appealed to youth.
She said the aim of the competition was to get youth working with a top designer to make their idea become a reality.
"It's a wonderful process that puts a young, upcoming artist in with a top designer, and the result is amazing."