Hi there ,
One of the requirements is to submit the source code to a GitHub repo so it's viewable, but it doesn't have to have an open source license. If it's helpful, here's a snippet from GitHub's Documentation:
You're under no obligation to choose a license. However, without a license, the default copyright laws apply, meaning that you retain all rights to your source code and no one may reproduce, distribute, or create derivative works from your work.
I am not a lawyer, but I always recommend adding a license - open source or otherwise :)
Cheers,
Lee