Are you referring to Itch.io's game page editor?
If so, I just use the (¶ Format) > "Code" option from the toolbar of the main text editor.
If you mean within the actual jam submission form?
Then I don't think that's possible, as it's a simple multi-line text box - so it'll just show the raw text.
(Which is fine, as it's intended for me/others to be able to copy + paste your code to verify the entry, if needed)
Hope this helps 🤓