whatever OS you're on this should work, go into you're OS's text edit software (notepad, text, and text edit,  as an example for windows, chromebook andmac, I'm not sure about linux) copy paste this into it:

<div id="scratch-games" class="container">
<iframe style="margin-top:-76px; margin-left:-17px;" allowtransparency="true" width="980" height="820" src="//scratch.mit.edu/projects/embed/url number here/?autostart=true" frameborder="0" allowfullscreen></iframe>
</div>

at the end of the url, there will be a number, here is an example (https://scratch.mit.edu/projects/11111111/) copypaste the number  of your project into where it says  url number here  (after  erasing  it, of course) you can mess around with all the other variables in this script but I would suggest not as that might mess up the entire thing

then save it and name it "text.html" (the .html being so that the computer knows it's and html file) then  at the  itch.io upload page  change the "kind of project" section to say html then go to the "upload project" section and upload the html file, then click the save button and test. you'll need to post it on scratch the post it on itch.io then submit it to the jam

also I would suggest posting a scratch game you've already made on itch.io to test it.

if you need help just reply to this comment and I'll help you to the best my abiltiy.

I hope this helped you, and happy jamming :)