If you're using Scratch 3.0, there's this website which converts an online project to an html file: https://sheeptester.github.io/words-go-here/scratch3-htmlifier/

I found that if you make the game on the Scratch website, there is an option on itch.io to add an external file where you can put the games URL in. This basically turns the Download button into a play game button

Heres what I did specifically on my game

1. Set 'Kind of Project' to Downloadable

2. Select 'Add External File' (it's to the right of the 'Upload File' and 'Choose From Dropbox' buttons)

3. Change type from Executable to Source Code ( I don't think this step really matters, but just in case)

4. Copy and paste the URL for the game into the 'http://' section

Hope this helps!

Turbowarp packager can turn it into zip files for windows and mac