From what I remember from my CB200 (which, while not a similar bike, was, as with lots of Hondas, put together in a similar way), you have the part you are holding (called a 'spring hook') 180 degrees out. It should loop
over the stand pivot or stand (just above where your thumb is in the photo). But I think I can just make out evidence of where the hooks inside curve would have rubbed against the stand itself. Hook it over the stand pivot, and you may see there's probably a boss on the frame, facing out, which you slip the large hole in the hook onto, then slide it so it can't come off. At least, that's how my CB200 was, so I expect your 250 will be somewhat similar.
It looks like the spring hook has actually worn, rather than corroded, as even though it's not round when new, the hole in yours is very much elongated.
BTW, you need a new chain!
