The Project Spark beta is far from new at this point, but it's no longer restricted to Windows 8.1 users. Some 250,000 people have signed up and I imagine many more will express their interest now that the beta can be experienced on Xbox One as well. Do that here, if you'd like.
Note: if you already have a code for Windows, you're golden. Just follow these instructions. I haven't yet touched Project Spark but looking over this montage, people seem to be making great use of the provided tools. Shout out to the person keeping the StarFox dream alive.