The Wachowski brothers created the movie, The Matrix. Before the movie was released the brothers created a world for the concept of the Matrix. They themselves created different forums for the movies as well as a video game based on the movie. The two brothers had the concept of the movie's story being told through different mediums.
Other stories have followed suit, such as the Harry Potter series, although, they are on the opposite end of the spectrum from the Wachowski brothers. Warner Brothers studios made the Harry Potter books into movies. Then when the movies became popular, they made forums, video games, and merchandise for fans. The book The Tales of Beadle the Bard was a story written by J.K Rowling and it was based off a book that was referred to in both the book and movie series but the book was not about the characters specifically. The book is considered a transmedia story because it is another medium of the same story.
The Wachowski brothers is a good example of their story being told through transmedia because they themselves came up with the concept of their story being written in many mediums. J.K Rowling wrote the Harry Potter stories but she did not create the other mediums that came about because of the popularity of the movies.
Both movies follow the idea of covering the stories in many mediums but only do the Wachowski brother's truly follow the idea of transmedia.