By toggle do you mean you want the screen to be locked from rotating when tilting the device in general? Itch.io can't do a thing about that, you'd have to go into your device settings and turn off the automatic screen rotation. If you mean making itch.io fullscreen on your tablet without the border at the top so it doesn't rotate that way... I'm actually not sure. I assume you're on browser, I don't think there's a mobile app for itch.io. Maybe check your browser settings to see if there's any settings to make the site mobile friendly, or gesture keybinds to toggle things like fullscreen, something like that?
Hope any of my suggestions helped. I'm not too sure what problem you're having, so I addressed multiple.