If you go to the "edit game" menu on Itch and tick the "enable scrollbars" checkbox in the section labeled "embed options," players will be able to scroll on the embedded version.
If you're comfortable editing html in your browser via right-click + inspect, people can do this on the page directly. Before you open the game, right-click on the "Run Game" button, inspect the element, and find this:
scrolling="no"
Change it to
scrolling="yes"
Now scrolling will work when you press the Run Game button.