There are different possibilities to add a scratch game. One is to download your Scratch Game and then convert it to an SWF using Junebeetle's Online Converter. I usually use the settings "Autostart" and the double resolution, that is 960 x 720. Then you get an SWF file which you can upload on the itch.io game page.
Another possibility is to make a file with html code to directly display the game via the Scratch Webpage. This article describes this, but the explanation is in German language, sorry. The screenshots and html code snipped can be helpful nevertheless, I think.