So there are a couple of ways you could do it.
You could use an online repository such as Github or BitBucket to upload your project files, and then you can just provide a public link with your submission or on your games page directly to the repo.
Another way would be to zip up the project files themselves, and then just upload them onto the itch page as a direct download.
There is also the option to record a video overviewing your project files - so you would open your game in whichever engine you are using, and basically just summarise how it is set up. You would need to display the entire script (you obviously don't need to go over it line by line, but it should be at a pace where the whole script is summarised over a few minutes depending on its size), and it would also be a good idea to discuss any third party assets used in the video as well. You can use OBS (which is free) to capture the relevant screen content as a video, and HitFilm Express is a good free option for editing/rendering the video.
I am also open to any other suggestions for how appropriate evidence of the project files can be submitted if you have any other ideas, but those 3 options I would say would be the primary methods used by most people.